maybe the mechanics you're going to are not porsche-specific mechanics?
i was referring to the rubber seal above the gas tank inside the hatch area under the carpeting, although the actual rubber seal around the hatch glass itself (or the hatch not fitting properly and causing a reverse/drag air leak) can cause this problem as well.
this is a VERY common problem, and there are dozens of possible causes. check rennlist for them (search for: exhaust fumes).
here are just a few of the dozens of threads;
http://forums.rennlist.com/rennforums/showthread.php?t=270169&highlight=exhaust+fumes
and;
http://forums.rennlist.com/rennforums/showthread.php?t=267119&highlight=exhaust+fumes
and;
http://forums.rennlist.com/rennforums/showthread.php?t=266133&highlight=exhaust+fumes
also, find a better mechanic who knows about porsches, not just any joe-blow redneck wrench.