[viper4321]

hey everyone, sorry if this has been discussed my searches didn't come up with much, or maybe its the key words i'm using. haha. anyway. mines a 07 335i coupe, recently i noticed an odd issue. the car has a cold air, brittleman exhaust, and an AA piggyback tune from the previous owner. i've had the car for about 15K miles, been great. the other day i went into stock map on my tune just for the hell of it which i have done just after purchase to "feel the difference" however this time on acceleration (only hard acceleration) the car felt like crap, like a pulsing/ surging feel from the engine, rough etc.. turn the tune back on and its back to normal.. tune off crap again. obviously the tune is masking something, the question i'm trying to answer is what?
on further diagnosis i T'd in a boost guage (i dont have one permanent yet lol) and did some pulls, interestingly wile the turbos seem to have plenty of output, with the tune off i can actually watch the gauge fluctuate 2-3 pounds on steady throttle, explaining the "pulsing" im feeling. with the tune on its far more steady and putting out what id expect.
kinda a long read, sorry. but if anyone has some thoughts on what the tune is masking id appreciate the help.