Im having problems outta my 4l60e in my 2000 gmc z71 its shifting fine until it warms up then it starts slipping. when i turn the truck off and start it again it shifts fine for a bit then starts slipping again. im unsure of what the problem is but i could use some info it has roughly 213,000 miles on the truck unsure of the trans.

 

Sounds more like an issue with the transmission going to a limp mode where the computer is putting the trans into a fail safe mode. The reason I say this is because of you cycling the key and it seems fine for a bit. I would start with having your vehicle checked for codes first. this might be a simple fix despite the mileage you have.

 

I had similar problems to this with my tranny. Its really a common problem with 4L60e’s. I found this guide at this site that lists the top problems and solutions to fixing 4l60e’s. Here is the link, maybe it can help you too:
http://www.twincharlotte.com/transmi...r-Charlotte-NC