Bad computer? Vacumn Leak?

I have been having that EGR issue for some time now, but recently my gas mileage dropped to 22 mpg! Even stranger yet, I accidentally left the interior light on one night (effectively killing the battery) so I brought the battery out of the cold, and put it on a charger inside the house. The battery was okay, but when I started the car, the SES light that continuosly randomly lit up (Always the EGR code 32) didn't come on, and stayed off for about a month! During which time I was averaging at 35mpg (I've got up to 38 at times, but not often). However, the SES ligh slowly began to revert to its original problems, and the gas mileage has dropped significantly. So, out of chance, I pulled the battery off, let it sit for fifteen minutes, put the battery back on. Amazingly enough, the problem stopped...for about a week, and now still persists. What kills me is the incredible drop in gas mileage. I want to end this problem for good, and was wondering if it is a computer problem. I've swapped the computer out, but the EPROM was kept with the car as the computer came out of a different car (87 olds), and had a different EPROM. I am thinking that is the problem, but not sure, and was curious if anybody had heard of that being a problem before.

Also, the heater stopped working sort of. The Vacumn controlled valve won't open to allow heat in, and lets in a 50/50 mix of heat and cold air. I can push it so the heat is on fully, but slowly reverts to the 50/50 mix. Is there a vacumn leak causing that? Under a heavier acceleration, the valve that switches between heat and defrost will also "center", making some air come out the defrost vents, and some to come out the heater, and it bugs me. Full brake fluid resivior, but it won't suck the fluid back very fast (giving a brake dummy light) until I let off the accelerator. Does that sound like a vacumn leak?

If it helps, the SES light only comes on during a slow acceleration.