BozaLS1

02 z28 ls1. only mod is headers. So im driving and 1st shifts to 2nd then shifts to 3rd but doesn't grab til mid rpms and then won't shift into 4th. Anybody every have this problem before? I called a mechanic and he told it may be the tranny filter and fluid but wouldn't all my gears be messed up if it was that? 4L60E transmisson