If its at the point where it plays up at the moment, take it for a ride with the tank "open" but taped over. So it should not play up if the breather is actually blocked, but, I thought they where a pressurised system?
It could be that the valve in the cap has failed and does not allow the tank to adequately pressurise when the fuel quantity gets low. I could be completely off track but it might be worth a go? Sounds very odd. Best of luck and keep us posted when you find out what is causing it.
"I'm a thousand miles from nowhere,
Time don't matter to me,
Cause I'm a thousand miles from nowhere,
And there's no place I'd rather be!!