Hi Adams,
I have the same problem, have also been to 2 Urologists. One said it's a psycological issue and the other after all checks and physical examination, prescribed Viagra.
It's like, when I use Viagra (which I've done many times), everything is normal and I can go on with my girlfriend for hours, but when I'm not on Viagra, my erection suddenly drops!! during foreplay or when we're about to remove the clothes for intercourse during foreplay.....it's very embarrassing.
Currently i'm experimenting with homeopathy, will let you know if I come up with a cure. You do the same and let's keep in touch!!