Whats the deal with Mod motors and weather?

I have noticed that my Mod motored mustang seems to be more at the mercy of the weather than most cars. When its hot out the car runs like crap. When its cold out the car runs like the proverbial raped ape. I know all cars are somewhat at the mercy of the weather gods, but it seem like the Stang more so than most. Is it the tune that they come with? Is it the type of computer system they run? People who have had their stangs tuned; do you notice that the car runs better in the heat than it did stock?
Whats the deal?
 
Thats what Im talking about. Once the temps. go under 70 the car rocks. It just seems like Mustangs are extreme when it come to the weather. Other cars I have had didnt seem to be so influenced by the weather. They ran worse in the heat and better in the cold, but not as extreme as my Mustangs reaction to the weather change.
 
Vegas sucks for our cars. I drove through Vegas on the way to CA, and was also in Vegas 2 weeks ago and my car ran like absolute crap. No power, and would even ping a little. I was so glad to get out of Vegas. Not only is it hot as hell there, but it is also like 2000ft above sea level. Bad driving conditions all around. However, I love the non car realted activities in Vegas.