Originally Posted by davetibbs

Based on the large amount of research I've been doing, this is almost certainly bushing wear in the stator/E Clutch, which is a very common problem for the 6HP transmissions in these cars. I have an issue when if I drive my car really hard/get the transmission fluid hot, I can get it to "jump" through the gears from 4-6, and from what I can tell faults with the E Clutch generally are most apparent in the higher gears (e.g. 4-6).

It's of course possible that it's just a faulty solenoid - and I posted a thread on solenoid/Zip Kit installation here, but for me while that maybe firmed up the shifts slightly it didn't solve the problem in the higher gears, so I currently have a spare transmission that I'm rebuilding with uprated clutch kits. I'll post a thread with details when I'm done, but it's definitely not a job for the faint hearted though!

I suspect your transmission will need a rebuild to replace (at least) bushings.