1999 Chevy Camaro shifts hard

The car drives and shifts fine for 2 or 3 weeks then it shifts real hard at the first shift change . It will do this for the rest of the day or maybe two, the check eng. light comes on .Then shifts return to normal and a few days later the light goes out.When this happens I let off gas at first shift change to lessen the thump that occurs.I tried lucas tran. treatment and it ran fine for about a month then it started again. I also replaced fluid and filter.I hope there is something I can do myself to remedy this problem . Thanks