APR chip=CEL, now only 5PSI, help!

Sorry it's so long, but please read and help! I have a completely stock 2001 Audi A4 1.8t quattro and got an APR chip installed by Achtuning in Redmond,WA. The car ran great for a few weeks, held 15-18PSI and pulled really strong, until suddenly it threw a CEL. Now the car will only boost 5PSI! If I reset the ecu the car shows no CEL and holds 15-18PSI again for about 5-10 minutes, then the CEL comes back on and the car goes back into this 5PSI safemode. I took it back to the shop and they said it was the cam positioner sensor, which I paid them to fix, and the problem still exists! APR told me to upgrade to an >03 A4 dv which I did, but still no fix! Achtuning told me the code being thrown is a "voltage code" but they don't know what to do to fix it. What should I do?!? I'm thinking it may be the coilpack, because of the voltage code, any suggestions? This 5PSI mode is driving me nuts!!! PLEASE HELP, and merry christmas.