Craftsman 5.5 hp lawn mower revs up & down/run erratic?

My Craftsman 5.5 hp lawn mower Model No. 917.377700 has begun to run erratic; that is, the engine revs up and down continuously (this model is designed to operate at constant, non-variable RPMs).

I checked the carb choke linkage/springs. They seem to be working like they're supposed to, although the spring on the inner butterfly valve's linkage is very fine/delicate.

I checked the carburetor (Keihin brand--float, jet/nozzle, needle valve, bowl, springs, action--all seem to be working as intended).

Air filter OK. Spark plug OK. Crankcase oil OK.

Fuel delivery system OK.

Any ideas would be welcomed and appreciated.

Thanks!

Calli Pegian
March 2010