If just flipping to reserve fixes it - I take it you didn't open the cap the second time since you were moving - it's not a venting issue.
If there's no kinked lines, I'd suspect there's some debris (rust flakes or some other contamination) in your tank and after a while enough gets trapped on the screen to restrict fuel flow. Switching to reserve changes where it's sucking gas from, so it works. When you switch back to on, the debris has moved away since there was no suction. If this is the case, I'd suspect if you left it in reserve for 75 miles or so (maybe less, since reserve and prime suck from the bottom of the tank where more debris is likely), the same thing would happen.