i have a 99 silverado 1500 with a 4.8l vortec, I'm not a big fan of it, but when i have a friend in the truck he will ask me to step and it, and when i do it will shift into some low gear and rev into like 5000 then back to 3200 then back to whatever gear it left 5k at. strange...
Also, it was around 20 degrees when i was driving home and my transmission started slamming into gear, so i parked it and waited.... not a problem since! Would really like to know what the **** went wrong, im sorry but i don't like it when my pickup tries to pop a wheelie going 10mph...

 

Try changing the throttle position sensor. My 1993 started shifting extremely hard under very light throttle. Brand new transmission and had it checked by a certified GM mechanic was not the problem. It worked fine for awhile then would start doing it again. Finally got a code for the tps sensor and changed it out 2 years ago and have not has that problem since.