I skimmed through all the comments and didn't see anybody say what I'm 99% sure the problem is. It happened to mine, sounds like exactly the same thing. It'll jump out of gear (4th/overdrive) on occasion. Gradually mine got worse and worse over a year or two's time. Turns out the 4th gear hub goes out on these cars quite often. Eventually if you're like me, you keep driving it and eventually you just lose overdrive because the gears get "stripped" so I was told. My advice, is if you plan on having the car for a few more years, get the trans rebuilt, and make sure they put the hardened 4th gear in. The shop I took mine to said they wouldn't put the oem one in anyway, but ask to be sure. That's my advice and my opinion on the problem. If you want more people's opinions, check out mnclubgp.com like someone said earlier. A lot of the people on there know a lot about these cars and modify them to 300+ wheel horse. In other words, they also know a lot and might be able to help diagnose the problem.
