RE: new plugs= less power
Holy crap, and I thought South Dakota was cold right now. You guys over in Minnesota are getting hit a bit harder, aren't ya?
I hope you're using a thinner oil like 5w-30.
EDIT: Sorry, didn't address the spark plug question. I know a lot of guys here like their Autolites, but I prefer to use either Champions or NGK plugs. I am currently using NGK Iridium plugs. A little expensive, but I could feel the difference slightly when I replaced the original plugs that were in there. (Although that was probably mostly due to the fact that the plugs had not been touched for almost 60,000 miles and not so much the special, super duper magical iridium.)
If I ever have to replace the NGKs, I might just use Champion's Truck plugs, Champion Platinum plugs, or something along those lines.