I have been running my car in SD mode for a few months now due to very low MAF readings (i think the MAF is going out), and I have not had any problems to speak of until now. My overall fuel economy has decreased slightly, but i believe that may be due to the colder weather, yet now if i leave the car undriven for more than a day or two, when i go to start it, it will run for approx. 2 seconds and then die. When i start it again it will idle a bit high for a second and then be just fine. The car runs fine otherwise, and you can see my mods in my sig below. Has anyone else encountered such problems? Is it due to running in SD mode? or are my o2's going out? I have not thrown any O2 related codes. Any info or ideas would be greatly appreciated...

 

Time to hook up the wideband.
Assuming no vacuum leak, you probably just need some small adjustment to your open-loop coolant correction table for warm-up.
It could be just a little lean at the very lower temps.

You're right that gas mileage drops during very cold weather. Warm up takes much longer, and denser air requires more fuel.
A V8 passenger car is hardly underpowered, so the cooler air & added power capacity are not needed during normal driving, and so gas mileage drops a bit.