My 1980 CX500C has been losing power while running, apparently not getting enough gas. Choking it helps somewhat at that point. When I pull over, open the gas cap, replace it and set back out, it runs with full power again for a while before the problem recurs. I have checked the gas cap vent - air blows through it freely. So I replaced the cap's O-ring, believing it was maybe falling too low on the tank fitting, blocking the vent. But that didn't solve the problem. Has any one experienced this? Or otherwise have any insights?