1997 Chevy Suburban Sputters when going up hill

I have a 97 Suburban. It runs fine around town and even on the freeway. I noticed when needing to get on it for a lengthy period, like going up a steep hill, it will begin to sputter and loose power. After letting off the gas for a few minutes everything will return to noral. I thought it was bad plugs or wires so I replaced them. It still does it. Any ideas???