AC / Heat / Idle Issues 1989 Mercury Grand Marquis

OLD PROBLEM:
I had a suspected vacuum leak in my Grand Marquis as when I accelerate
the AC would cut out. Idling is a little uneven and sometimes will accelerate
faster than it should. I found a vacuum leak at the hose connecting the
MAP sensor but patching it didn't completely solve the problem. I cant find
other leaks but I think they might be there. I tried using carb cleaner and
replacing as many vacuum lines as I can but no joy.

NEW PROBLEM:
I have since replaced my heater core as it previously had a leak and had
been capped off. Now I have heat but no AC. I'm not sure if the heat is
overpowering the AC but with the blower on it is blowing hot air even with
the temp set to minimum. Also, when accelerating heat starts to flow
through the vents even with the blower off.

Using an OBD 1 diagnostic code reader I get an error "EGR Valve not
opening properly". Could this be the problem or could the error a symptom
of a vacuum leak elsewhere? The EGR valve was replaced a couple of
months ago.

Is it likely the same issue as before with the addition of heat into the
equation or have I added a separate issue while replacing the heater core?

Any help would be greatly appreciated.