So here's the deal.I have a 1996 2.2 5 speed outback.Car has 206k miles.I've owned since 2005 when it had 120k.I am a mechanic and have maintained the car myself.Did the second timing belt event at 195k.This included water pump ALL idler pulleys and tensioner.Also all accessory belts,coolant hoses an spark plugs.The car has always ran fine and gets 25-30 mpg.For the last couple of thousand miles its had an intermittent problem to include missing,erratic idle sometimes stalling.The car will restart after 5 secs.or so of cranking and then might run like a top for a day or two.When the problem crops up it's mostly at a steady speed and feels like a bad miss.I originally thought fuel,so replaced the fuel filter.Drove the car problem came back within a few miles.Figured 200k miles enough life out of the fuel pump so replaced that.Again ran for 30-40 miles then same problem.My local Subaru dealer mechanic(a friend) said it was probably electrical and the bosch platinum plugs I installed were the problem,so I installed original spec.ngk plugs.Again it ran well for a short time.Since then I have replaced the coil,ign.module,plug wires,cam sensor,crank sensor,throttle position sensor.These were done one at a time to evaluate the effect.The odd thing is when it is having the problem,you can put the clutch in and rev the piss out of it and it might "solve" it for awhile.Also the idle speed will sometimes fluctuate from 2000 rpm down to 500rpm or so.This led me to believe the throttle position sensor.The check engine light has been on for 70k miles and indicates oxygen sensor bank one.I have a code scanner and no other codes ever show up.I don't think it can be the o2 sensor,but that or the mass airflow sensor is my next guess.I have a donor car that was wrecked,so all these parts are easy and free to try.Just wondering if I'm missing something here?

You're throwing parts at a problem that you don't fully understand. Is the CEL on? If so what is/are the code(s)?

Which O2 sensor code?

This can certainly cause rough running and misfiring. The ECU could be adding/pulling fuel to compensate for false reading from the sensor.

 

Another option is the painfully simple vacuum leak.

When it loosed power if you take your foot off the gas does it idle just fine?

 

If so, the MAF sensor is going bad.  

 

Just fixed a friends 96 Impreza that would "die" at random.  It was loosing the MAF signal.  

 

Got a used Subaru MAF from the yard, problem fixed.

I was going to sat bad maf, ans it will misfire and stumble horribly, like you will have to feather the throttle just to get it moving. Also clean or inspect the IAC if the idle does not settle down right away

My 95 at 210K miles started to stall randomly.  Then it wouldn't start for an hour.  It would then start and run OK for 5-30 miles and stall again.  It was the MAF.  I replaced it with one from the junkyard and it's been OK for 10k miles.  It didn't set the code until it stalled for the 6th time.

 

So the old MAFs are a problem. 

I've replaced at least 5 MAF sensors that would cause random stalling. I've not seen one cause high/hanging idle problems, or constant misfiring but its possible.

Generally its caused by a cracked solder joint inside the MAF, when the ECU loses MAF signal it kills the engine. No stumble no fuss, just quits. If it happens while driving it'll feel like you put the brakes on. A couple seconds later you'll get power back and continue as if nothing happened. It seems to be repairable if you are adventurous enough to cut the top off the MAF and have some experience with a soldering iron.

as I said check engine light has been on for 70k miles.Always has code for o2 sensor bank 1.I can reset light but it comes back after a couple hours of driving with the same code for o2 sensor bank 1.I am going with the MAF.As far as throwing parts at the problem,it's only because I have a donor car and the parts are free!!!!!The fuel pump I installed was new since at 200k the car deserved one if only for preventive maintenance.