Need some help on my 2005 GMC Sierra Z-71. The power is lacking when driving. Truck has to down-shift just to go over an overpass. Not towing or hauling anything. Only mods are a K&N filter in the air box and cat-back flowmaster exhaust. Have changed plugs and the ones I took out looked fine. Truck has about 80,000 miles. Any help on this without re-gearing and buying a programmer? Any ideas will be greatly appreciated!

Chop