Finally fixed my limp mode on accelerating above about 2500 rpm, here is how. Car is a 2001 TDI New Beetle 5 spd manual which was 100% original equipment when I got it. If your wondering why I spent so much dough on this old car, its been in the family for 16 years and I have a goofy attachment to it. I love driving the damned thing. Just some background on this confusing problem. Same as all over the web, black smoke on hard acceleration an intermittent P1556 charge pressure negative deviation code , fading power, and limp mode. As you may know, the vac system on this car has to be working correctly as so much depends on it. New turbo (old one blown), new oil lines to / from turbo, new valve cover, new intake manifold, new EGR valve, new MAF, new air filter, new fuel filter, replaced ALL vacuum lines, verified integrity of vac line tees, new brake booster hose to vacuum pump (including new check valve), verified 2nd check valve before N75, new N75, new N18, new PCV valve, cleaned snow screen. Malone stage 1 tune (before I realized I had a boost issue). I used ridiculously expensive VW parts for N75 and MAF on the recoomendation of a local guru who confirmed that many of the knock-offs are crap. Had several vacuum leaks which were resolved. Ok, so I though I had covered everything in looking for the boost problem. It went away entirely for about 2 weeks after I had added some silicone sealant around the pipe from the vacuum pump (which goes into the brake booster line) to the right of the valve cover. Never had the sense to do a vacuum test at the time. But being persistent, I took a recommendation from this site and disconnected the vac line to the vac reservoir and connected it to a Mighty Vac guage, engine off, then started up. I read I should expect > 25 inches of vacuum, and got 18 slowly creeping up to 25 on the nose, but wobbly. Wondering about the vac pressure creep, I reached behind the large hose from the airbox / MAF to the turbo inlet and wiggled the tubing where tees off of the (new) brake booster line. To my surprise, vacuum dropped to 10" and varied with wiggling the line, returning to full vacuum in some positions. Now many recommendations on the web say the vacuum pump outlet pipe can rotate freely, that it has an o-ring inside (I've never disassembled it). Other recommendations advise using only flexible sealants around this pipe (which I followed). Others say they have used epoxy. The correct approach is doubtless to disassemble the the vac pump and smack this pipe in with a hammer to seal, or buy and install an entirely new pump. Bottom line is that 3 separate attempts to seal around this pipe (2 with high quality silicone sealant and 1 with grey RTV) left a connection which leaked when wiggled, resulting in intermittent vac leaks. I removed the brake booster hose from the vac pump, cleaned off all the crappy glue (I also managed to eat through a coolant line by dripping this solvent based sealant on it, but that's another story and another self-inflicted fix job) with engine degreaser, wiped thoroughly with alchol and carefully sealed around the vac outlet pipe nipple where it leaves the pump with quick set epoxy from Harbor Freight. Yes, I am aware that a higher quality epoxy would work better but this car has me eating canned soup. Allowed to dry overnight (with block heater on) and reconnected brake booster line. Wiggled and no more vacuum leak. Cannot reproduce limp mode now, no P1556 code and good power throughout rpm range. Still have a bit of smoke on hard acceleration which I attribute to my Malone stage 1 tune and some blow-by given the age of the engine. Still have a small oil / soot leak to the right back of my valve cover, left of the vacuum pump, between the intake manifold and head. I can't see in there to source this, but I'm thinking they re-used the intake manifold gasket (not sure why there would be oil there unless returning from the turbo).