I am 53, 5'8",162lbs. Two or three times a week I wake up with a full erection(and its not because of a dream). Or I can get a full erection when looking at X-rated Movies. But most of the times I can't get a full erection with my wife. When I am able to get a full erection with wife, I have a hard time keeping it. Why?

I think for some reason the penis muscles relax while I am sleeping. Allowing the penis to fill with blood. As for x-rated movies, that is extreme stimulation. I love my wife and feel terrible that I can't get or keep an erection.