I've had the same thing happen w/my car which was purchased in March '01... I live in flatlands, where I didn't experience this at first. But the first time I went for a drive up in the local mountains, it happened. And it continued to happen intermittently when I'm going down a hill decelerating, foot off the brake, just coasting, between 2.5k and 2k rpm in 1st...
After some time passed, it would happen in 2nd gear also. In addition, it would happen on flat roads when I had more than one person in the car, the more weight in the car, the more likely to happen.
So I took it to the dealer from which I purchased it, because their service department has a well earned reputation for quality work. I knew that they had seen the insides of more than a few impreza trannys.
However, this dealer, dealer "A" I'll call them, weren't able to diagnose the problem, because I dropped off the car, the mechanic would take it around for a drive by himself on flat roads, and couldn't replicate the problem. Also, this dealer has sold many wrx's and foresters in the last few months, and so their service dept was booked extremely tight. Which meant that the service dept told me to wait for the local regional SOA service rep to come by, and then he and I could drive the car to diagnose the problem. sigh...
But at the same time, a new Subaru dealership sprouted up in the neighborhood near my office. After they had been operating for a few months, I spoke w/the service manager, and told him in detail the problem w/my tranny. He told me to bring it in, and he would have me ride along w/the mechanic, and he told me where there is a small hill near by which we could drive down to help diagnose the problem.
So I scheduled an appointment for a couple of days ago, and we were able to successfully replicate the problem. Yeah!
Yesterday evening, I spoke w/the svc mgr, and he said that they phoned SoA, and SoA has a tech bulletin, or something like that, about this issue. The dealer was given instructions, and a list of parts which would solve this problem. Unfortunately, they do have to drop the tranny and crack it open to fix this problem. The good news is that according to people in another thread who have had this service for this problem, it solves the problem.
The problem did not go away.. During the 19 months in which I have owned my wrx, it probably got a little worse.
The key is to follow Glenn's advice above... you need to spend a little bit of time determining the exact conditions in which the problem occurs. This may mean going on test drives yourself to determine in what conditions this phenomenon is most likely to occur. Remember that you won't be driving your car when the dealership diagnoses it, so you need to be able to give as exact instructions as possible, so that another driver can replicate it.
There is a thread on this same topic over in the tranny forum, which I believe was started before this forum was born.
It is called Tranny Jumping Out of First Gear
, and is at http://forums.nasioc.com/forums/show...hreadid=130776