Audi heal thyself(?)

[shaun d mullen <mullens@phillynews.com>]

I have put nearly 70,000 original-owner miles on my 99.5 A4QM Avant (30vV6) 
with no problems whatsoever other than having to replace each of the 
low-beam headlamp bulbs.  Then I recently and suddenly encountered what I 
would describe as some drivetrain juddering (my word) which could be felt 
through the accelerator pedal.  The Check Engine light also came on, but 
some time after the juddering began.  The engine seemed fine at idle and 
when revved in neutral, and the car performed satisfactorily when pulling 
through first and second gears.  But in third, fourth and to an extent in 
fifth, I felt the juddering when I would give the car a lot of pedal, 
especially going up steep grades.  The juddering more or less disappeared 
when I would back off the gas a bit.  I ran perhaps three full tanks of gas 
through the car (yes, I drive a lot) and then yesterday afternoon when I 
started the car and pulled away, the juddering had completely disappeared 
and the car seemed to be running absolutely normally.  I've driven another 
100 miles or so since then with no problem.  The Check Engine light remains 
on.

I always buy 93- or 94-octane gas from one of several high-volume stations 
(Amoco, Sunoco, Mobil), which would seem to diminish the possibility of the 
problem having been bad gas, but might the juddering have been symptomatic 
of that?  Or was it another, possibly drivetrain-related problem that the 
car's ECUs diagnosed and dealth with?  Could my car have healed itself?

Comments, advice, shared similar experiences appreciated.

Shaun Mullen
Wilmington, DE