Advice needed; S6 engine problem

[Steve Marinello]

Okay, Guys...

So I've got this "new to me" S6 avant, complete with RS2 turbo, manifold,
injectors and an, as yet undetermined modded ECU (assuming MTM/Hoppen).
Running smooth and strong for the first couple of weeks.  Then it starts
intermittently "gagging" on itself.

Pull up at a stop light and it's like it's missing on one or two cylinders,
then crawling power only coming away from the light, which was luckily only
two blocks down from the dealer, which is where I was headed for another
reason.  Get my friend the service guy out and we repeatedly rev the engine
and determine that it doesn't seem spark related, as it doesn't go away at
higher revs, and are scratching our heads when it just disappears and
smooths out.  Strange.

Two days later, I'm headed home across the bridge.  Suddenly become aware of
a strange resonance/vibration throughout the car, as if you're riding over
soem bad stretch of road, but it can't be the road.  Try accelerating and
realize, the engines crapped out again.  Downshift and try to rev it out; no
go.  Proceed ahead (at the 9 mile mark on a 24 mile bridge) and realize that
it's getting worse, but at least I'm still moving.  Nasty smell seems to be
around.  Afraid I'm going to destroy the thing, but there aren't any real
options at this point.  A guy in a pickup pulls alongside and motions me to
roll down my window.  Says my car exhaust stinks to high hell, but it
doesn't seem to be smoking.  Says it just started about a mile back. (No
kidding!)  I continue like this for another 10 miles and...it just
disappears again.  All smooth; full power.  Off the bridge and heading home
on the backstreets.  A little over two miles away, here it is again...and
then it's gone two blocks from my house, only to reappear as I pull into the
driveway.  Unplugged the O2 sensor to check that and it was either unchanged
or worse.  Sweet rich smell to start, immediately replaced with gagging
sulphur fumes.  I'd had a pretty bad headache the last two days, so this
could have been going on at a lower level for an extended time.  Normally, I
don't get headaches.

Having had a problem with water in the tank of the '85 urq recently, this
possibility is also there.  Add dryer to the tank and try to drive it out
again. No dice...until in sight of my house again and it's gone.  It was
gone for the next three days until heading across the bridge this morning.
It showed up five miles from the south shore.  However, this time I
downshifted into fourth and brought the  revs up and then backed off a few
times and it sputtered/spit and the problem was gone again.

Been great weather lately.  Relatively dry for New Orleans and mid 70's to
mid 80's.  Shell Premium in the car; filled up the day before this started,
however.

Any ideas or BTDT's?  Fuel, sensors, cats?

TIA,

Steve