Premature Problem...but It Doesn't Happen All the Time

My girlfriend and I have been dating for 2 months now. She had wanted to have sex on the second date (she had thought all guys wanted that, but I didn't), but I said no at the last second, and told her that I wanted to get to know her, and actually form a relationship.

3 weeks later we had sex for the first time together. It was incredible. I'd say minus foreplay (I usually give her a massage to get her started up), it lasted about 15 minutes. Since then it was fine, we even went 3 times in one night. She told me she really enjoyed it, and even brought it up the next day how great it was. A couple of times there were times I never even had the desire to ejaculate, but just kept going until she was worn out. I felt like a machine!

A few nights ago we did it again. 5 seconds after I entered her I felt that I was about to come, so I stopped, then started again slowly, repeating this process once or twice. It lasted long enough for her to climax, but just barely.

Today we did it and I almost came right away again. We changed positions and it helped a little, but again - it lasted just long enough for her. Just after she climaxed, I did.

I'm starting to wonder what's going wrong - before, I was doing great, now I seem to be falling apart. I talked to her about it, and she told me I was doing just fine, and that I shouldn't worry about it.

I had this problem with my last girlfriend, as well. Sex was great for a while, then I came early a couple of times and she would ridicule me, get mad and go to sleep. But then again, it would come and go (no pun intended), sometimes I'd be able to go for what seemed like all night, sometimes not even a minute. Is this normal?

Also, when I masturbate, I can make it last as long as I want, pretty much. Sometimes a half-hour, sometimes 5 minutes, so the problem doesn't present itself there.