Definitely see a vet.
Sheba would sometimes fall over when her heart problems were acting up, before we had her on a beta blocker (propranalol) to regulate her heartbeat. The vet explained it to me that her heart was beating too fast for it to maintain that pace, and would occassionally skip a beat or two. When that happened, she fainted. She usually got up a few seconds later, but her heart could have just as easily stopped for good.
I'm not saying that's what's going on with your cat. Only a vet could tell you, after a physical exam and probably bloodwork. But it's something that needs to be checked.