I have recently bought this car with 22,500 miles on it. I runs great except it annoys me as it pulsates and surges slightly when holding at any steady speed between 45 and 55. It doesn't do this at 70 or any speed below 45. The car is basically stock except for ATR ram air kit and K&N filter, pit bull chip, 160 stat, ATR stage III kit (whatever that consist of?) and throttle body intake coolant bypass. I am aware of these mods because the previous owner had the documentation in a folder. I bought this car from a classic car dealer. Any ideas on where to begin looking would be appreciated. PS. It does seem to idle a little rough but I did'nt know if that was unusual for these cars.
Next time you feel it try riding your brake just enough to release the convertor clutch. You are right at about the speed it engauges. GM called this chuggle and is considered normal. An aftermarket prom may raise engaugement speed.