Ur-Quattro

[CLeiter@aol.com]

Help, I have an 83 Ur-Quattro with the IA computer mods. The car has a
stumble at around 4500 rpms with the boost at 13 to 14 lbs. Now with the
boost at 12, the car does not have the stumble unless the air conditioner is
on (but its not as fast as I want). I want to crank the boost back up, but I
need to take car of the stumble. 
Its seems as though the computer is reading an over boost and is shutting
down the fuel pump. I've checked all the sensors and replaced the air temp
sensor which helped alittle bit. Also, when I disconnect the manifold vaccum
line from the back of the computer the car runs great. 
Could this be do to the modified exhaust system. I welcome any ideas.
Thanks in advance.
CLeiter