I would defineately replace the filter first.  You may have some water in the
bottom of the tank.  However I had a similar problem with my '84 Vanagon.
Every time it got down to about reserve it started bucking and running like
crap.  About the same time I noticed a leak of gas at fill up.  What I found
was that one of the 5mm lines that attach to the overflow tanks up in the
front wheel well had gone bad and was leaking.  I replaced this line and the
leak was gone.  But as a side benefit the low fuel level bucking also went
away for good.  I didn't replace the filter or anything.  Why would this leak
make the van buck?  I have no clue.  I asked the list for possible ideas last
year but no one seemed to have any ideas.
Ken Wilford
Van-Again
John 3:16