PROBLEMS with my radiator

I have an 86 Toyota Celica GT that has been overheating...

Here are the things I have tried:
-Already replaced the cap
-put more antifreeze in (too many times)
-changed a leaking gasket

Basically the car runs fine...but when I do a lot of freeway driving or just long drives and the engine starts to get warmer...as soon as the car slows down the engine starts to heat up...

I noticed that the radiator has no problem dumping fluid into the reserve, but it doesnt seem to pull it back in because the reserve is always full even when the radiator is low...but the radiator still has pressure when i unscrew the cap...even a day or two later after being run...

I am not a car person...i am a computer person...but if anyone has any suggestions...i know car people that can help me...I just need to let them know what needs to be done...


thanks