Avant - Audi A6 :: 1999 - Occasionally Stumbles / Misfire And Throwing CEL Code

We have a 1999 Audi A6 Avant (wagon) with about 190,000 miles. Best car we've ever owned, and want to keep her running until the wheels fall off (so to speak). We split our time between sea level and a ski town at 7000' (yes, that may be relevant).

Last spring our Trusty (sea-level) Mechanic repaired the exhaust system; both tailpipes were shot (car sounded like a tank). Rather than replace with Audi parts (was going to be a full exhaust system replacement - $000s) they spliced in some new tailpipe. Car ran quietly after that.

However, immediately thereafter (like, on my drive home from Trusty Mechanic's shop) and ever since the car will occasionally stumble (misfire) and throw a check-engine light. Typical scenario is after having been off the throttle while going downhill and then tipping into the throttle at the bottom of the hill, the car will stumble and on comes the CEL. Stumbling lasts 5-10 seconds then gradually subsides. Light stays on, though. Not 100% reproduceable under those circumstances (but doesn't really happen under other scenarios), but happens often enough to be disconcerting.

We recently decamped from sea level to spend the summer up in the mountains (great bicycling), and while the car misbehaved a couple of times on the way out of town, by the time we got to 5000 ft or so, the CEL was no longer illuminated. The car has not misfired once while at altitude despite plenty of situations that would presumably have triggered the issue, were we at sea level.

Trusty Mechanic has replaced some injectors, some coil packs, and the spark plug cables. No behavior change.