[someguy000001]

I have this same problem.
There are a few posts about this one but I'll sum up for you. Lemme guess.. was the code 1870?

Common problem with 4L60E transmissions.
Caused by Worn valve body
Can be fixed with a Transgo shift kit.
You could try installing a transmission cooler, as the problem only happens when the tranny gets over a certain temp (200 i think someone said)


Maybe BlazerLT or Mikado will pop in and help out. They'll know more.