We have no problem running at highway speed with a full tank but once it drops down to about 3/4 tank, the RPMs drop and we have to get out of traffic. We had the vapor filter and charcoal canister which fixed a problem filling the tank where it would spit fuel back out when filling but we still have the hesitation problem. Stop and go city driving doesn't seem to be as effected as much so we had thought we had a vapor lock problem with the fuel tank. But we have no issues filling the tank now. Any thoughts?