2 quarts a week!?

I would say that is headgasket territory. Check your coolant for oil and vice versa, and consider a compression check--although the car stalling when the A/C is on would indicate poor compression. Make sure that your A/C compressor spins okay and isn't siezed up either. Here is a DIY for the M103 engine, yours should be similar

PeachPartsWiki: Valve Stem Seal Replacement

However as I said, I'd inspect or replace the head gasket as well.

__________________
TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth