1991 magna check engine light

hey guys
I recently bought a 1991 magna executive and had a problem with the
transmission. i replaced the transmission computer because it was
stuffed and that seemed to fix the problem. However i am still having
problems. after i have been driving for a little while, say around 8 or
10 kms the check engine light appears and when i am going up through
the gears it seems to slip out of gear and rev hard. i take my foot off
the accelerator and push it again gently and then it goes into gear.
this problem only occurs when i am driving in the city and am regularly
going up and down through the gears, stopping and starting. it doesnt
happen if i am on the highway because the car is constantly in the one
gear.

Please help!!

Luke