Transmission

My car does not shift into 4th (D) when I am going past 50mph or so.....when I let off the gas it goes down to 1k rpm. When I shift it to 3rd the rpm goes up to 2.5k (normal) and it looks fine. Is my transmission going out or can it be the solenoid actuator inside the transmission?

i have had that problem i can't tell you exactly what it was but in my case the torque converter lost the oil from inside it(it doesn't self wick very well) and because the converter lost the extra oil mass it would attempt to lock in and it would end up kicked out because it didn't have the inertia. I also once lost the actuator and not two days later i had to tow the car to a shop because the tranny dropped. I would say take it into a tranny shop and have them look at it and i would say have them rebuild it right away cause if they don't they will eventaly.