P1640 doesn't necessarily mean the PCM is bad. It's possible, but not very common.
You will want to look at your EVAP purge solenoid (rear valve cover, right side, has several vacuum lines/hoses going into it. Some may be marked "FUEL/TANK EVAP" or something similar + electrical connector) or the boost control solenoid (front valve cover, right side. Has electrical connector and vacuum lines...on the right side it will also have what looks like a vacuum port that has nothing attached. There is supposed to be a foam filter on it.)
One or both may be on their way out. I had a constant P1640 on my car. I replaced the boost control solenoid and then the light would only come on sometimes. I replaced the purge solenoid and it hasn't come back on since.
You can easily get the purge solenoid off of RockAuto (use ACDelco or Delphi parts). The boost control solenoid is a little trickier...it is nearly impossible to find one for a Series I supercharged engine. HOWEVER. You can get the one that goes on the Series II supercharged engine. All you have to do is bend/modify the bracket attached to the solenoid to make it fit. Everything is plug and play after that. I have one on my car and it works perfectly.
Another distant guess is the torque converter clutch solenoid. This is the other thing that quad driver controls. If you have no shifting or transmission problems, consider looking at the previous two solenoids first.
Supercharger boost control solenoid:http://www.amazon.com/gp/product/B000QIGLIG/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1
These are the two replacements I used on my car.
My thread for reference:http://rivperformance.editboard.com/t11144-p1640