95 SE Problems

My son has a 1995 Pontiac Grand Prix SE 3.1L with 124k miles on it, he has alot of highway driving to do and he has been saying at about 55-60mph the car seems to surge forward and shifts sporadically all over the place, what would be the root of the problem? bad solenoid? bad tranny? any i mean any suggestions would be appreciated, thank you

sounds like the TCC is not holding and causing it to slip in and out of lock-up.