I have a 2002 5.3 z71 silverado. First the truck runs excellent for the most part. In the cold starts up perfect and everything.. But when i drive about 10-30 min it losses power. i can't pass anyone. Can barely climb any hills and it just misses and backfires but never stalls out. If i pull off the road shut it off for about 5 min start it back up i can peel rubber if i wanted to but 5 min down the road its doing the samething again.. Any clue whats causing this??