with Sonny. This doesn't sound like a clutch at all. I would also check to see if you are getting good vac at the advance of your distributor. I have seen vac line suck down and pinch off when under a load (high vac) this would cause timing issues under load making the engine lose power. A clutch would be a waste of time and money. If the engine was recently changed the clutch was probably yarded out as well.
Just my $.02