clutch mystery

i love my 1993 900s(298k miles---90% highway). several weeks ago, when releasing the clutch, the clutch pedal exhibited a 'pulse' at the top of the pedal arc. the 'pulse' could not be felt when depressing the clutch---only when releasing and only near the upper 1/4 of the pedal arc. interestingly, the 'pulse' could be felt with the engine running OR at rest! this 'pulse' was intermittent for a week, went away and is now a permanent 'feature'. now, the 'pulse' has gotten more pronounced AND, rarely, the clutch grabs very unexpectedly hard and nearly stalls the car. pedal pressure seems fine. thow out bearing has been making noise for years and does not seem any louder. shifting is fine. no fluid leaks. please help! thanks, dean