1 Lucky Texan

not a thumping? wondering about lower control arm's rear bushing.

likely underhood on an 04, I think earlier models had it on the charcoal canister? 2 wire connector and 2 small evap hoses. maybe near strut tower/firewall? I saw some folks mention it could be near the alternator? sorry, that's all I got from quick search.

purge valves have moved around with model years, but yeah, you have one.

does it also happen parked? seems like heat shields could be a possibility.

more searching online seems to point to the purge valve; https://www.youtube.com/watch?v=ZP5gPigl6NQ

lol, not like I know what any of the results would mean.... between the purge valve and the evap 'drain' valve (connected to the carbon canister) the fuel tank should not have any pressure differential with ambient air so, I think the air moving effect you get when removing the cap 'could' be a clue....?

try a squirt of starting fluid immediately - when it's running rough? at least we'd know if it was fuel-related...? or, next time you feel it would fail, do a few OFF to ON (not START) key cycles. wait for a 3 count at ON, then go to OFF, back to ON, 3 count, etc. - quickly. That runs the fuel pump and should fully pressurize the rails....? you said you can detect a little air movement...wonder what would happen NOT at fueling, but say right now you removed and replaced the gas cap? would it start smoothly?

First time we went to CoSpgs in the wife's Outback, it was weird to park between 2 other Soobs. Here, (back then), that could never happen. Up there, there are as many Subarus on a city block as in a zipcode here.

eh, the debris thing is a stupid guess really, imagine how much anything like that would slosh with normal turning/braking....

the cap/o-ring problem is not too bad to inspect for, I just dunno if the Forester's design is similar. I may not know exactly what the issue is here - but can't help wondering a coupla things; suppose the input of fuel is disturbing some debris in the tank that collects on the 'sock' - but temporarily ???? any whooshing of air when the gas cap is loosened/removed? and sense if air is moving in or out? I'd be tempted to try 2-3 tankfuls with Techron (maybe seafoam?) or similar gas treatment, maybe some odd check valve is sticking somewhere???? maybe scan the ECU and see if a code is pending?

pull plugs when it's cold - before any start attempts, see if any are wet with fuel. maybe cylinder wash-down is part of the problem?

manufacturers consider something like a quart in 900-1200-- miles a problem. A qt in 3000-5000 would not meet warranty-levels of concern I don't think. you might try a new PCV valve. You might send a used oil sample to Blackstone. maybe a leaky injector is diluting the oil or ????? but 750, yeh, you could call SOA, explain to a regional rep that you already documented a concern in the past, and now experience a very high level of usage. Maybe you can get another test done but, at this age, who knows what Mother Subie will do??? I'd prepare myself for disappointment.

plug looks good to me - minimal residue. I realize it is xtra work, but axles are L-R exchangeable, then, if the turning noise moves with the axle, that should exonerate the diff right?

if you go down 25, and if you have time for a 'side trip' take 12 out of Walsenberg to Cuchara, then to Trinidad. awesome scenery.

Akro, if you decide to replace the trans, do a search of the forums and internet about seating or retaining the torque converter. If it slides forward, it can cause issues when bolting in the trans. Possible cracked hydraulic pump.

France, might be tricky.

yeah maybe the switch or the 'button' on the pedal? some models have an issue with the button falling out...my brain doesn't recall what generations have that lol

any exhaust system damage? you may need to look at live data from the ECU

Dave mentioned wires, you say the car has been sitting, you should look around for rodent damage I think too.

ABS is very sensitive to system voltage. Some (maybe not all) of the symptoms point to a charging issue and, knowing that the H6 has a special alternator, I can't help wondering if it is failing or, if aftermarket, could be the wrong style. I thik getting a voltmeter on the battery terminals or elswhere and checking/monitoring voltage is a good first step. A fully charged healthy battery will be around 12.6v, charging can range up to over 14.4v, maybe 0.2 - 0.3 more. But the ECU in that car can hold charging current back. are there any aftermarket electronics in the car? CB radio, woofer amps....etc.?

I wouldn't swap 08 with 01s. Tell us what problem you are having.

????? I like the ground cleaning you have planned. Throttle Position Sensor could have a 'dead spot' in its mid-range sweep. though I wouldn't overlook the possibility of more than a single issue given you symptoms. could be battery cable is corroded under the insulation. Possible bad Knock sensor - cheap easy to swap, if bad, often the plastic housing will be cracked. automatic transmission? aftermarket axles can cause vibrations when stopped but in gear, goes away in P or N - seems weird, but it's a legit problem for some folks. I suppose, if the car is missing at idle, you could squirt some ether start or even an unlit stream of propane from a handheld torch into the intake.If it smoothe-out, could be a fueling problem. Have you measured the system voltage?

tell us about the starter replacement. Did the original fail a bench test? Is the new one a parts store rebuilt? Did the replacement ever work?

another longshot - clean or replace the crank position sensor. Sometimes metallic debris collect on it (often from failing bearings - looking at you Toothed Idler).

others can comment, and actually, they have a lot more useful info that I ever will specific to that engine. But, looking at the big picture here; consider this a totally new project. Perhaps converting back to all VW is an option? Or, converting to a Subaru EJ engine?