95 bonny, check engine light and shifting problem, please help

I have a 95 bonniville wth about 160,000 miles on it... for the last couple months the check engine light has been on and since then when taking off from a start its very sluggish, almost like its in 2nd gear or somthing. once it gets going its fine downshifts up shifts everything is fine.. anyways i disconected the battery to reset the codes on the check engine light, and it was off... well when i do that it runs from the start fine, has plenty of power but wont shift from first to second... now at about 3000 rpm in first gear the check engine light will come back on, and it finally shifts, but then its back to being sluggish from the start again. any suggestions? ive changed the fluid and filter, and also took to autozone to do a diognostics check, but their obd2 reader wouldnt read the codes.. thanks

bump

My transmission went at 175,000 so I would suspect that yours is getting ready to go. When mine went all I had was 1st gear.

The engine light could be almost anything.

Take it in to a shop and have them run diagnostic tests to see what is the matter then decide if you want to fix it or let it go.