Shifting problems

Hello, my name is Chris for Edwards my trucks has a 5.9 360. Having problems with shifting. Truck does not have hesitation going into gear or taking off. Does not have a problem shifting from first second but when it shifts into third it waits the revs pretty high it also seems like it does not slip as much as it falls out of gear when you let off the accelerator to kind of hold a steady speed it falls out of gear if you accelerate goes right back into gear with slight hesitation. But will hold the gear as long as you are accelerating. Transmission has been rebuilt with in the past fifty thousand miles and was rebuilt correctly this is just a recent issue. I have been having a buddy of mine say that it could be a shifting solenoid or solenoids in the transmission I've done research and I cannot seem to find if it is a one to solenoid to three solenoid a shifting solenoid or a pressure solenoid. I am at a loss of knowing what to do I am pretty mechanically inclined so I am not sure ignorance level that I am not understanding. I just cannot figure out which solenoid I need like I said transmission operates and functions properly up until about ifty five to sixty miles per hour it does not so much slip and falls out of here if you accelerate hard it goes right back into gear with just the slightest hesitation. If I could get some help on this I would be grateful. Thank you.