2005.5 Limp mode, sluggish low end, maf code

I'm stumped, hoping someone much smarter than me can help diagnose this issue.
2005.5 Jetta, 250k
A few weeks ago, car went into limp mode on the interstate. I pulled over, disconnected maf and noticed considerable improvement. I replaced with a questionable spare I had and I ordered a reman maf from local parts store and would install when I got home a few days later. Before I got the new part, car started going into limp mode pretty frequently with maf disconnected or connected.
I got home, installed the new one and there wasn't much improvement. I ordered a new Bosch and returned the reman. After installing the Bosch, the problem persisted. So that's 4 mafs, 2 used, one reman and one new Bosch. I think I safely ruled that out.
In addition to limp mode, I'm also getting sluggish acceleration off the line, not as bad at highway speeds. I'm still getting the Emissions Workshop! warning with my cel light. I can always get out of limp by restarting. Some days the code clears, or at least light and warning go off, but still sluggish off the line.
The only code is P0299 Underboost and the MAF code, although I think that is cleared.
I replaced air filter, fuel filter and the N75 just to rule that out.
My mechanic, quite capable but not a TDI specialist checked vacuum hoses, and any signs of an obvious leak, but suggested I take it to a dealer since he feels a little out of his element with this. The only gurus I know in this area have moved.
Can anyone point me in the right direction for anything else to check before I ruin my lifetime streak of never doing business with a dealer?
Thank you.

Hmmm last year my car went into limp mode and I had a MAF code for a while due to a hole in an intercooler hose and then it bursted when I accelerated and put car into limp mode but I’m sure you’d hear that? I could hear a pretty audible swoosh noise

Yes, no weird noses and we pretty well checked for any cracks or holes.
My mechanic did a little more digging and thinks it might be worth replacing what he calls the boost controller or wastegate. Would that be the same as the vnt actuator?
If so, it sounds like that might not be available or replaceable for this model?
Can anyone confirm or dispel?

Sounds like the turbo act need rigged. Try the zip tie mod and see if that helps.

Replaced the actuator last weekend. Not a fun job. Cleared codes and noticed improvement for a few days. Then today cel back on, Emissions Workshop! and limp mode again.
Not sure what else to check?

Check to make sure you EGR cooler is not leaking vacuum. Had this happen to me and it would go in to limp mode.

VagPhillips said:

Replaced the actuator last weekend. Not a fun job. Cleared codes and noticed improvement for a few days. Then today cel back on, Emissions Workshop! and limp mode again.
Not sure what else to check?

Click to expand...

I suggest checking ALL the vacuum components, including the vacuum reservoir. I had a limp mode issue that only occurred when I was driving in the North Georgia mountains above about 3000 feet. It turned out to be a leak in the vacuum reservoir that is integrated into the cylinder head cover. It was a simple fix with RTV around the original glue joint on the cover.

I've been enjoying the cooler temps driving the higher altitudes of the North Georgia mountains all day.
That's a good tip, haven't checked that.
Over the weekend I was going to eliminate a code by replacing a glow plug. When I removed the harness it nearly disintegrated in my hands. I didn't have much illusion that this would help my Emissions Workshop! Code and limp mode though. I replaced the harness and bad glow plug, also used maf cleaner on my very New Bosch maf.
Something worked. Codes we're cleared and car running great for a few days now.
I wish I hadn't cleaned the maf because it would have very interesting to see if replacing the bad glow plugs harness helped although if it did I certainly couldn't understand why.