automatic transmission shift points?

Hey guys.. i wan wondering if anyone knows when my eighty eight should be shifting around? it usually shifts around 2500rpm.. the reason i'm asking is because i brought it in a month or so ago because the check engine light was on. mechanic said the code was showing transmission slipping. so they wanted 500 just to look at the transmission to see if anything was wrong. I decided not to and the check engine light went off 2 days later. Now i'm not sure if its just in my head that the car doesnt seem to be shifting right away or if the transmission is actually slipping..

any ideas?

thanks!!