So my Crossfire Coupe Limited accelerates really slowly most days, it feels like i'm driving a 4cyl. I replaced my crankshaft sensor recently and after unplugging the battery and resetting the throttle my car ran like a dream for a few days and there was a dramatic increase in acceleration, after awhile it went back to slow and sluggish unless i'm driving with a HEAVY foot.

This week it's also been very cold here compared to what it normally is and has averaged 15-25degrees. It is now stuttering to crank, it never completely stalls but takes 2 or 3 seconds longer to startup, once the car warms up it will shutoff/crank normally. My car has 70k miles on it and i'm not sure if it's ever had a tune up (I bought it with 56k on it).

Does it sound like a tuneup would fix these issues and if so how much will a tuneup cost? $300-$500? I'm not too car savvy so it's going to the shop for whatever it needs.