[Wookiee]

I have a '70 and when I got a non-vented gas cap it acted just as you described. I got the right cap and thought it was fixed but still had to replace my fuel pump because it was damaged by the pressure. So, yes, this will cause your problem. Like others have said, you need to unblock your vent tube for the tank or buy a vented cap. Your mileage on fuel pump may vary, but don't be surprised if it's damaged too.