140 transmission questions

[rchase]

I got a "really" good deal on a 1999 S320 with 99K miles on it. I was in a bit of a hurry to get the deal taken care of and failed to get the car inspected in the process. After a few weeks of driving I notice some "quirks" in the transmission and suspect that I don't have long before I need a rebuild or a replacement unit.

The transmission has done some odd things some I have read in the manual are normal some are definately not normal. At first the way the car held the car in low gear when it first started freaked me out until I read in the manual the computer does that to warm up the emissions system. From there I have had a few other "oddball" encounters. One time out of the driveway when shifting from reverse to drive there was a delay. I pressed on the gas and the car lurched forward when the gear engaged. The worst one and the big confirmation for me was my experience on the interstate today. I was going about 75mph and this guy in a Lexus was going to attempt to pass me and cut me off. I accelerated and the transmission dropped down a gear and got "stuck". I was holding 4000 rpms pretty steady and the car would not shift. I removed my foot from the gas and the car still would not shift. I switched into neutral and then back into drive and still could not get the car to let go of the gear it was in. I eventually pulled over and put the car in park and shut the car off and turned it back on and everything was normal.

I talked to my mechanic this morning and they indicated that this was common on these cars and normally the onboard diagnostics would flag it and complain on the display in the car. Other than these two events the car has been almost flawless. I have noticed some odd shifts but Im not sure if its me being overly sensitive to the issues with the car. I have yet to get any display errors and wonder how long I have until this problem reaches critical mass and if I would be doing any additional damage by driving the car.

Advice is welcome especially from people who have had a rebuild done. I do intend to have the car repaired because even with the out of pocket cost of the repairs the car is still a wonderful car.