1994 Cadillac Seville check engine transmission

Transmission problem
1994 Cadillac Seville V8 Two Wheel Drive Automatic

I just bought a 94 seville with 49000 miles and a week later the check engine light came on and when I put the car from park to reverse or reverse to drive it clunks and shifs hard into second but some times the light goes off and it shifts fine can someone please help