James - It sounds like your gas cap vent is clogged, or if you recently replaced the cap with a new one, it’s possibly not the correct vented type.
Alternatively, a new supposedly vented cap may be bad out of the box. It’s been known to happen.
On your tank vent question, you can see the upper opening by removing the gas cap. Small hole on the upper side of the fill neck. But to check the entire vent you need to pull the seat back to get to it.
Aside from these things, there are other possibilities that could involve your fuel pump, carb and even ignition.
I’d start with checking/resolving the tank venting first.
