I've had this same problem w/ my '02 4runner when I first bought it. I started having problems about 2-3 months after I bought it. Took it to the dealer, but they said they couldn't find the problem, and everything was working out just fine. 3-4 months later, it happened again and I took it the dealer again, and this time, they changed the floater inside my fuel tank and I haven't had any problems since.