I had the same problem in a Datsun -- not knowing the feed mechanism inside
the VW tank, maybe my experience will help.

Turns out a rubber grommet of some kind had been disgorged by a gas pump
at the local Texaco.  At certain times, the grommet chunk would wedge
itself between the feed tube in the tank and the tank bottom, shutting down
the gas flow.  I tried everything before I finally dropped the tank and 
washed it out, at which time I found the grommet.  Problem never recurred.

...Dave Phillips  '69 bus     |  Internet:  phillips@aries.tucson.saic.com