2004 Saab 9-3 Linear- Hates extreme temperatures- Overheating and no heat

I am stumped! Here is the problem:

On Dec 9th, we drove the car about 70 miles from Columbus, OH to Dayton, OH. This happened the first cold day of the year, about 22 degrees. About 25 miles into the drive the heat stopped working and the car started to overheat. We pulled over and checked the anti-freeze level, which was fine. I figured my thermostat must be going. We ended up limping the car home where I replaced the thermostat and voila fixed! On Jan 6th, we drove the car about 6 hours to Seven Spring, PA and back with no problems (keep in mind the temp was above 32 degrees). The following weekend we attempted to drive the car up to Michigan and made it about 20 miles out before the heater stopped working and the car started to overheat. This time it was 17 degrees outside, the first cold day since Dec 9th.

At this point, I figured that my protection level on the anti-freeze wasn’t adequate to keep the cooling system from not freezing. We ended up limping the car back home where I flushed the cooling system and replaced the thermostat again with a SAAB version ($50 vs. the $5 AutoZone one) while I was at it. Maybe I was unlucky with a bad thermostat. I drove the car for a week and kept having the same problem, no heat and it was overheating.

My thought at this point, water pump… Replacing the water pump on a SAAB is one of the worst jobs you can do (~$900). The water pump wasn’t leaking though but I thought that maybe impeller was just spinning free on the shaft and not pumping.

Just this Monday, the heat started to work again and the car stopped overheating (it has been consistently above 32 degrees). The car is now working fine, which means that the water pump must be fine… right?

I keep my fingers crossed that on the next cold day the problem doesn’t come back. You guys have any idea?

One side note which could be related: In the summer when temperatures are above 80 degrees the AC quits working. Under 80 degrees it works fine. I am thinking that the AC turns off because it senses the car overheating. Maybe I do have some issue with my cooling system that is related to both of these problems.