Car has a dead spot, need help!!!

[Jagdeesh Diocee <jdiocee@Northwest.ca>]

My Audi has been running great since I have owned it.  Last month, my
bro borrowed it to tow all his stuff back home. The distance is roughly
500 miles.  He towed a 5x8' UHaul, fully loaded, and the car fully
loaded.  Driving in 4th the whole way at 5000rpm.  Ever since this
little trip, sometimes when starting off in first gear (somewhere around
900 rpm), it hits a dead spot for a split second (like the car stalls,
but comes back).  I have no idea why.  It happened once since he got
back, but just now during lunch, it happened about a dozen times. Today
has been the hottest  day this year, about 27 celsius  (80 F).  I keep
the car in good shape, I'm a 'fix-it-before-its-broke' type of guy.
Would an incorrect mixture (too lean) cause this.  I am going to richen
it a bit before I go home and test it out.  Any other ideas??? 

84 4000s -I5, non turbo, fwd (179,000km)