[Biturbos4] MTM Boost ISSUE/QUESTION

[Witte, Michael J (HPFS)]

Installed a MTM chip this weekend  (on 2000 TIP). 

When oil is warmed up, and boost is requested, I feel, and see (through boost gauge) the improvement, delivering 15psi boost (temps around 30 Fahrenheit outside. Run 20 miles, running on cruise control, switching cruise control off, boost won't get any higher than 6 psi. Detached ECU, reset and read VAG codes, banks lean and rich (indicating bad MAF, I know, its a 2000, MAF will be replaced shortly). Took another test-drive, no problems, on acceleration, easily 15PSI, kick in the back, butt dyno great improvement. On highway, whilst coasting at 70MPH, in fifth, accelerating to 85, again, boost up to 12-14 psi. Dropping speed to 60 mph, accelerating to 100 mph, 15psi. 

Fast forward, trip home, boost all the time, at 15psi, for 20+ miles, at last exit, boost won't come up higher than 6PSI (in any of the gears). Cruising through downtown, back to 2nd gear, boost runs easily up to 15psi....

Pressure tested, no boost leaks found, BPV's replaced with 710s less than a month ago. Any recommendations, suggestions, or does this chip has a mind of his/her own ?!!?