My boyfried, I believe, suffers this same situation. He gets a good erection but after some time, 10-20 seconds, he starts loosing his erection. One thing I found helpful in our relationship is talking about it, I know it is difficult for a man to aknowledge this but it has strenteng our relationship. One thing that he and I have tried is for him to perform for some time and then stop, then play some more and start again. In order words, he tries to avoid ejaculating. It is some kind of diffucult for him to do it but it has help some. Of course it is frustrating for me, but I love him and I think is worth the try.
I'm pretty sure that your partners have noticed this and if you haven't bring it up yet please do so.
One thing i'd like to ask you guys. Even though my boyfriend and I talk about this very frequently, I haven't been able to convence him to go to the doctor for a check up. I have read some article, can't remember the website since I have been doing a lot of research about this topic, that it could be some kind of infection to the penis or something on those terms. How can I help him understand the importance of this without hurting his self stem? You guys with your macho attitude is kind of hard to get through you!!!!!