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What would cause a turbo400 to disengage when I let up on the throttle? Other than that everything works fine. It's like it goes into neutral and then has to catch up when I accelerate.
Does it upshift at normal speeds? Is your issue with the trans happening in 3rd gear or lower gears? Need a bit more to go on.
Initially it sounds like a late upshift problem. Could be leaky vacuum to the modulator or defective modulator. Also could be the govenor or shift valves sticking.
I can't help you with your problem but I can give some advice. I had a work van that did the same thing except it did it in 2nd gear. The van was always loaded and weighed about 7500 lbs. so downshifting into second was a necessity at times. Called a local GM-Chevy garage and they said to bring it in and they would change the screen and fluid. I didn't think that would fix the problem, but I knew they should know more than me. I was quite disappointed when I got it back with the same problem. The only difference was my wallet was $100 lighter.