ADON996

I have a 1995 wagon wrx, it has a chip fitted (been told by owner its a scoobychip) also has a ind kit and full exhaust from turbo back. Now plugged in a boost gauge to check boost as i thought it didnt quite go as well as i expected. Low and behold it was only boosting at 12 psi then dropping off, since then i have checked and adjusted actuator, cleaned the boost solenoid and cleaned the hoses making sure the pipes have no leaks etc and the restrictor is still in place and cleaned the maf sensor. Took it out after doing all of this and still the same (a little spike more . . think due to re-tensioning the actuator)
I then put a MBC on and set it up and works a treat and will happly run 14/15/16psi. Does this sound a common problem that happens with the boost solenoid? i always thought they over boost when faulty rather that under boost? any1 else had a similar problem?
Also had checked for fault codes and nothing?
Thanks and sorry for the long post!