My 1990 Buick Reatta

Have you recovered from loss of drive? I haven't heard of many major transmission issues with the Reatta being common, and I have never experienced any with the two I have owned. Not implausible though. Firstly, I would check your ECM diagnostic codes, there are codes for specific issues with throttle positions, sensors, and gear circuits (you can find these easily with Google) If you have had a total loss of drive, but the other gears are acting as normal, you may have to replace the shift solenoid. This would be the first part I would look at---assuming you have done the obvious---checked trans fluid, pan dropped, filter changed, etc. If not the solenoid, perhaps the sensor. I suspect a minor problem, if not the solenoid, maybe a sensor. It is strange that you only lost the drive position, but perhaps no DRIVE is just the result of a bad sensor or solenoid...since you still have the rest of the gears in working order? If it is not an issue with a solenoid or sensor, transmission shop is the way to go, no doubt. Reattas very often have trouble with the fuel injection system. (This is to be expected with 30 year old fuel injectors and pumps if the car was not vigorously maintained.) My Reatta often, on heavy acceleration, would momentarily not move, and then lurch forward as you described. I was able to continue driving, however. Soon after I noticed this issue, my fuel pump went bad and I was left on the side of the highway. If you have ruled out a major transmission error, I would suggest checking out the fuel pump and any related issues with fuel injection. Fuel system should be easy to troubleshoot ( Not as easy to fix ) * A fuel pressure tester attached to the fuel rail should read 40-42 psi if you have a good pump and regulator, and the fuel pump should run 12 volts if your system is healthy. If you don't get those readings, you have a problem with your pump and / or the wiring to your pump. If you have ruled out transmission error and fuel pump error, test the Fuel Pump Relay (Terminal #1). The pump relay should have 12 volts. If you do not have 12 volts from the pump relay, the fuse #8 needs to be replaced. Good luck, lots of observation and guesswork here, but hopefully I was helpful.