Check the codes before anything else.
It may have a failing lockup torque converter which would explain the surging feeling without cruise control on - the TCM can detect excess slip of the torque converter and can throw the transmission light which can cause other strange behaviour as the transmission is in "emergency" mode.
My modified a340e in my 97 S90 would exhibit such behaviour after modifying the torque converter for a higher stall. If the transmission tried to engage the lockup while cruising at a greater than usuall inclination, it would occasionally carry on as explained above.
Codes codes codes!