Uncoupling Problem on Lionel Graduated Trestles

That's a tough question to answer, sight unseen.  It sounds as though the loco coupler is forcing the coupler extension on the car behind it down, thus opening the coupler on the car.  By coupler extension I am speaking of the piece of sheet metal that extends diagonally from the coupler.  This extension allows manual uncoupling using your finger.

Easy answer, you have to have an easement entering and exiting grades to allow the things to stay in reasonable alignment.  You say you only used half the graduated trestles, I'm guessing you skipped every other one.  That's a giant NO-NO, you are exacerbating the issue by making the transition even sharper than it normally would be.

I see that I wasn't clear enough in my description. When I said that I only used half the trestles, I meant that I only used the 4 or 5 smallest pairs. It seems that I have to place the 2 tallest trestles closer to each other than the normal 10" and place the next smallest set a bit further then the normal 10". That seems to lower the grade from the peak and help the problem.   However, I just don't understand why if I placed the trestles in order according to height that a normal 10" spacing for straight track does not seem to work well.

Well, the problem is still the same, when you crest the top of the grade, the transition is too steep.  This is a pretty common issue.