My Briggs and Stratton 375 has two speeds - flat out or stop!?

I have a B & S 375 on a Masport mower. It's always performed well, but on starting it recently after a few months layoff, it seems to want to run at two speeds only. Flat out or stop! When I start the engine, it immediately jumps to high revs, but throttling back simply causes the engine to cut out. No amount of juggling the throttle will produces a comfortable level of intermediate revs. I've lubricated the throttle cable to ensure it operates smoothly (it was a bit 'sticky' before) but the problem is still there. Does anybody have any suggestions?