Try removing the cap next time this happens. Or just try removing it when you shut the car off. If you hear an inrush of air, well, the vent is probably clogged.

Does this only happen when the tank is full? Worse when full? empty? The problem is almost certainly fuel related. Maybe when something gets warm under the hood, it loses its seal and allows air to enter the system. The car runs fine until the point when you turn it off and then it will not restart???

Jay.

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On the road, currently:
'83 300D (German spec.) 168k mi. - - Wolfgang (tucked snugly away for the winter!)
'87 300D 375k mi. - finally went over to "The Dark Side"
'87 300TD 225k mi.
Cannondale 66cm CAAD5 w/Campy/Jamis Aurora
Hujsak Custom Fixie/Landshark Track Shark Custom/Ahearne Custom 29'er