Need help with a 1.8T problem

[Michael McLaughlin]

I have a 99.5 A4 1.8T Avant Quattro that's been having some  
drivability issues. It first manifested itself as what felt like a  
really bad misfire. The car would stumble under load, and slowly got  
better. The first time it did this it cleared itself up and was fine  
for about a week. It did it again, mostly cleared up and then started  
doing it pretty much all the time. It never lit the check engine light.

I took it to my mechanic, who pulled codes and found one about the  
long-term fuel trim. He replaced the MAF, which fixed things  
initially (and he said the MAF numbers on the VAG-COM were fine) but  
it slowly got worse. Now the misfire isn't all that noticeable, but  
it does stumble a bit under load. What is very noticeable, though, is  
that after the condition returns the car feels like it's really low  
on power. It won't accelerate much above 3,000 RPM and going up hills  
on the highway, for instance, has become a challenge. It also tripped  
the CEL.

We took it back, he swapped out the MAF for another one (seems like  
they have a high failure rate) and also found a very slight intake  
leak, which he fixed. Same story -- car feels great at first, then  
slowly gets worse, doesn't like to accelerate and trips the CEL.

A couple odd things -- every time the CEL has come on it's been just  
a few miles since filling the gas tank. Also, the two initial times  
where it felt like a misfire developed were also within about 15  
miles of getting gas.

It's going back to the mechanic Monday. Any suggestions from the list  
about other things to check out? Or did we just happen to get two bad  
MAFs in a row?

Thanks,
Mike