95 Lumina 3.1- AutoTransmission "thumps at overdrive speeds

My 95-3.1 with aut transmission is now "thumping' when Im at highway speeds around 65-75 miles per hour, especially when starting up a slight grade..This problem is a new occurance...after I took to mechanic to have the rear exhaust manifold replaced (said old one was cracked in two) and also had radiator flushed and thermostat installed( temp was running very low in winter) now temp is steady at about 1/8 to 1/4 above halfway point, but now experience this very heavy shudder/thump from tranny..are they related? Also, SES light has always comes on when I drive above 50- 55 mph and top a slight hill..It NEVER comes on if I keep speed below 50-55 mph....is my torque converter shot or is this a EGR/VSS prob??? sorry for all the erractic info...

My 95-3.1 started this odd shudder and rapid surging ( nearly twice per second ) when the converter tried to lock up. I could hear a noise during this surging. I don't know if I would call it a thumping. The tranny shop replaced the converter because it couldn't hold during lockup. It showed signs of excessive slipping and heat. It works great now except it seems like it locks up kinda hard and fast, with a jerk. Does anyone know if the lockup is supposed to be with a jerk or velvet smooth. When I first got the car it locked up so smoothly I couldn't feel it at all-- had to watch the tach to be sure it was locked. Maybe that's why it went out...it was already starting to begin slipping.