1996 blazer wired tran prob help?

hi i have a 1996 blazer s10 the prob i'm having is the truck runs great the first time you start it in the moring and i get down the road and stop at a stop sign and then the transmisson stops shifting and i lose 2 and overdrive i just took it to the shop and they said the pcm was bad so they replaced the computer and i just picked it up and it has the same problem so i don't know what to do and if it matters the abs light is on i have already spent alot of money on this thing and i just bought it about 2 weeks ago please help