When I had a P1128 it turned out to be the MAF. I tried to clean it first, but it didn't help. Do you know if your vacuum lines have ever been replaced? It's pretty easy and much cheaper than a MAF. There are some posts with pictures of where the lines run, I replaced mine with 3.5mm silicone.
You can monitor the output of the MAF with Vag-com (VCDS). This is the easiest way to tell if it is good or bad.
I had the 1128 a few weeks ago. If I cleared the code, it would run fine for about 2 cold starts, then it would only boost 5psi. Shortly after the CEL would light. THrough all this, no idle.drivability issues.