2001 Buick Century Transmission shifting very hard

My Century developed an oil leak last year and I had that repaired. Since getting that done, when shifting from 1st to 2nd and 2nd to 3rd gears the shift can be so hard that it feels as though someone is dropping a weight on the floor. This does not occur all the time only when the car is warmed up, especially in stop and go traffic. Additionally, there can be a flutter at about the 50 mph range. There are also problems with the service engine soon light and ABS light coming on randomly. What should I do next?

J