I am twenty-seven years old an have been having difficulty maintaining an erection once I begin intercourse (with my girlfriend). I have no trouble whatsoever getting an erection during foreplay but once we begin intercourse I usually lose my erection within five to ten minutes. This includes oral stimulation as well. This is really irritating to me and occurs at least three out of four times. I do usually manage to maintain but it is usually after three or four attempts and a good two hours after we start. I really don't know what the cause is or what to do. I would appreciate any advice or possible explanations or solutions.