WARNING: This page is about a different car, the 2004 GMC Sierra and 2004 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
No 4th or Slipping 4th
Checks
Causes
Checkball #2, 4, 8 or 12
Valve body checkball in wrong location or an additional checkball is installed.