Transmission Questions

[MPolzin@xxxx]

Greetings again, 
After a complete engine rebuild, I finally have the engine running properly on VIN#4761. The engine now runs beautifully, so we decided to take it out of the garage and take it out for a spin. The engine performed flawlessly, however the transmission isn't shifting correctly, so once again I am going to have to park the car until I can get this worked out. The previous owner had warped the heads on the car, and when parked the transmission was supposidly just fine. The tranmission fluid was drained and replaced within the last couple of months. Here is what happened...

I took it out about a week ago for the first time and I noticed right away that the tranny seemed to be jumping around inbetween 1st and 2nd at low speeds (around 25 - 35 mph). I didn't want to damage the transmission so after about 1/2 mile I turned around and headed back to the shop. On the way back the erratic shifting cleared up and I drove it for over an hour with no issues at all.

We finished up some final work on the car this week (new alternator, belts, installed a set of stainless fuel lines, and did some maintenance on the steering rack) and I took it out again last night. Initially all seemed great, no problems shifting through first, second, or third and I brought the car up to 55mph. I then brought it up to 65, and all was good and about five minutes into it the car started shifting between 2nd and 3rd erratically this time. 

I tried it again this morning and the same thing...If I shift the car manually (start in 1st, shift to 2nd, then 3rd) it will stay in 1st and 2nd fine, but once I go into third/Drive it starts shifting back and forth between second and third. It seems when I left off the accelerator it will tend to do the odd shifting, but it will do it occasionally when just driving along as well.

Very disappointing, I have hundreds of hours into the car and have spent almost $10K to restore and put the car back on the road. I'm good with engines, but drive train is another thing. Can anyone give me some guidance on how to trouble shoot this, or where to start? I know the shift computer on these cars are notorious for having problems, is there a way to determine if that is the problem or if it is something internal?

Thanks,
Mike Polzin
VIN#4761

[Non-text portions of this message have been removed]