What happens mate?
Does it sometimes boost really well, then drop off and go into like limp mode, and then shartly after boost again??
I've had similar issues, sometimes boosting upto about 18psi, then holding at about 14. (Oettinger / RSD map ) but sometimes it boosts well, and i can then feel it holding back, and the boost drops.
Then sometimes it'll only boost to about 10psi.
I'm going for a new N75 i reckon!
Check which one you have, there was a 'C' version on ebay the other day for 99p.