The main serpentine belt should last atleast 60k and, from my experience the intermediate or jack-shaft one about 40k & finally the supercharger one around 20k. Are you changing them just because there are some cracks or do you wait until small pieces of rib actually start to come off ? On my supercharger I've been successfully using the Dayco poly-cog belt, because the ribs are pre-cut, allowing the belt to wrap around the small pulley without further cracking. I also have a '94 and one other thing I can think of is the condition of your belt tensioners & pulleys; because the sealed bearings in the pulleys will dry out with age and cause resistance to the belt. However, that normally becomes obvious because of the noise and, then it's time to change them. You can get replacement bearings, but most people just buy new pulleys (the Ford Cobra set works - except the SC one is slightly larger and may require the next size belt, like a 393). The other thing I would suggest you check is your harmonic balancer - observe the smooth rotation of the big pulley while the engine is running. If the main bolt has cracked, the small pulley bolts become loose or the balancer itself become fatiqued then it will wobble some.