Sounds like a lift pump issue. Does it happen if first running tractor of after it has been running for done time. If it accrues after tractor has been running for a time if could indicate a tank vent problem as well. Next time it happens loosen fuel cap to see if it makes a difference. If it doesn't I would check the lift pump or kinked fuel line. Check the easy free stuff first.
It varies, first couple of times it did ok for a while, then got worse. Last time happened pretty quickly (although it was running while my neighbor tinkered with it). Couple of guys at work also thought it was a venting issue, so I will try it with a loose fuel cap to see how it goes.