2000 elantra cruise control

Alright, my girlfriend has a 2000 elantra with auto and anytime it is put into cruise, at any speed, it begins to surge. The surge is not registered on the speedo nor reflected in the tach. The auto trans has been replaced with a remanned unit, the trans brain was also replaced in an attempt to fix this yet it still happens. They claim that no code is thrown but it still surges when cruise is set. Unfortunately the surge is intermittent so of course it never happens when the dealer rides with us. The car is still under warranty so I want it fixed now. We've had it in the shop 6 times for this and they can't fix it. What do you all think it could be so that I can tell them and hopefully get it fixed?

I have this same problem with my 2000 elantra. I took it in and thankfully they felt the surges they ran all kinds of test on it and cant find anything wrong with it. To my frustration I have learned if you disengage the cruise control and let it stay off for about a minute or two and then cut it back on it does fine. :disappoin