I have a 2000 Chevy Z71 silverado with the 5.3 vortec V8 with about 195000 miles. When I start it when it's cold it acts like it wants to cut off. Then it will rev up to about 4k rpms for about 6-7 seconds then spit sputter then rev back up again. This is the 2nd z71 I've owned of the same body style and same motor. I have no clue what's going on. ANY input would help!!!