1999 Mitsubishi Eclipse automatic transmission

Transmission first started to act like it was slipping. Primarily coming down a steep incline or bridge while in cruise control. Never going up a steep incline and not while driving without using the cruise control. I fushed the entire transmission with Mobil 1 fluid which showed it was for my transmission(replaces diamond sp III). Also changed the filter. Car ran fine for over 2 months and started doing the same thing again but also on flat surfaces the rpm's would ramp up and down while driving as if the transmission was slipping, This would not happen constantly but every now and then. Finally one day it cleared up for the day and the next day I drove about 2 miles from the house and on flat ground it started slipping constantly and wouldn't shift gears. I pulled to the side of the road, shut the car off and started it again, put it in drive and the car wouldn't move at all in any position including reverse. Had the car towed to the house. I was told it could possilbly be the transmission control solenoid. What's your opinion?