Lawnmower keeps stalling, Why? (more info below...)?

It periodically stalls after running well for awhile; if I tilt it forward or bounce it up and down, when it starts acting up, it will rev up again. Sometimes it keeps running, sometimes it dies again. After it dies it won't start until I take off the gas cap and blow into the tank. It then runs ok until the next time. This is a 8 year old Sears 6 Hp "one pull start" mower. I've replaced the carb and cleaned out the fuel tank and lines but that didn't help. Out of ideas; has anybody else had this problem?