I have a '92 5.7 TPI with the 700R4 Tranny and have been experincing the tranny shuddering around 70 mph when coming in and out of the overdrive. I went to a mechanic and he said that this was a known issue with GM tranny's and sold me a $9 dollar bottle of "Shudder Fix." Well it worked for about a month and now it is back. I serviced the tranny (changed the filter and the fluid) and it still does it. Am I looking at a full rebuild??

 

It may be the converter. Apparently there is a seal inside related to the lockup that can fail. It has something to do with keeping pressure in and it makes the servo that engages the lockup just keep firing, which makes it shudder. I will bet if you unplug the lockup, the shudder will not come back. However, the overdrives don't like to have their lockups unplugged because it makes them get hot at highway speeds and cook the fluid. That's just my guess at the problem, but I had the same thing happen to my pickup with a 700R in it.