hhoowwiiee

I have been doing research for over a month now, and I could use some advice on my 98 Forester manual transmission. In April, the speedometer stopped working altogether, but the car drove fine at first. Subaru dealer here in Indy said that they diagnosed the specific problem as a stripped plastic part inside of the transmission. The cost to repair was four figures, I can't recall but maybe $1900. They suggested I drive with a GPS as my speedometer and save the money. A few days later, I had my first engine shake problem, after slowing down to match traffic on the highway. I took it back to Subaru, they said they could not recreate the problem. So they have not been any help since then. So for 3 months, I tried to fix the intermittent stall and shake problem. The shaking and stalling is intermittent. I can go for two weeks with no problems at all, then they come back. The stalls are more frequent that the shakes. The stalling usually happens when I am going downhill, or when I am putting in the clutch to downshift. Obviously the CEL is on, and I get the codes PO500, P1507, and P1540. I am told these are speed sensor, idle air control (failsafe), and speed sensor 2, respectively. (Autozone) I have had the following work done this summer, at several different auto shops. early June - Auto shop 1 cleaned Idle air control valve, and changing out arcing spark plug wire (Ran good for two days, then it slowly went back to Intermittent stall and shake.) late June - Auto shop 2 replaced knock sensor because this was the first code that came up after resetting all codes. This did not help at all late June - replaced dirty air filter on recommendation of Autozone tech (because of P1507). Helped overall operation, but did not fix the stall and shakes. Early July - Auto shop 3 cleaned fuel injectors. Helped overall operation, but did not fix the stall and shake. Late July - Auto shop 3 diagnosed IACV needing replaced, but they did not replace it because to me, it wasnt the high idle problem that members have descrbed, instead it was a low idle problem if anything. Also, that part was cleaned by Auto shop 1 in June. Auto shop 3 replaced the Neutral Safety Switch at my request. I thought this was the problem based on some forum posts from ferret, here on the boards. This did not solve the stall and shake. At this point, I did even more research on the message boards, and there are some posts that say the engine computer needs the speedometer code, and if it doesnt get it, then the IACV failsafe code gets triggered, along with the appropriate speed sensor codes. Does anyone have any insight on the cheapest way to proceed? The discussions here have been helpful for me, but now I need to zero in on my specific symptoms and get some input. At this point, I see two paths. 1. Fix the speedometer. Either bite the bullet and open the transmission, or install an aftermarket speed sensor and try to get it working with my speedometer. (Is this second idea even reasonable? It seems a lot cheaper, but is that going to work long term?) 2. Change the IACV. I can get one from ebay, used, but tested. Its a good car, 181,000 mi before speedometer quit. But its old, and I don't want the expense of the transmisssion work if I can avoid it. I would like to try the aftermarket speedometer fix, but I have never done this, and I will likely need to find somebody to install it for me. Thanks any advice you have. Please let me know if I need to be clearer with my descriptions. Howie from Indiana