i have the same problem. im 21. i used to get erections like crazy, and now i think i have mentally built up too much mental control over them and dont allow myself to relax enough to stay hard. I was watching "Talk Sex with Sue" if you are familiar with the show, and somone called in asking about this same topic. She said it is very very very normal and common. She said that it is natural for the penis to go from hard to semi hard continuously throughout intercourse due to the blood vessels trapping and then releasing blood. It is annoying though, i take some supplements sometimes that i think help a little, but i think the biggest thing is in the mind. also going without cumming for a few days helps too.