I had exactly what you're describing at 32 weeks (second baby) - I was having them for about 3 hours in the morning at about 5mins apart and then they let up for awhile and started up again that night. Really scared me, it felt like the real deal to me.
I mentioned to my OB a couple of days later and he checked me (I have a history of early labour) and baby was still high and I wasn't dilated at all. So they were BH's even though I could have sworn they were real.
He also said BH are much more intense with subsequent pregnancies.