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!