check valve lift. A cam lobe may have worn out, happened to mine with similar symptoms. Also had worn valve guides and exh valves leaking on the weak cylinders, and very weak spring pressures. It would seal when turned by hand for measuring leakdown, and it would seal enough for a compression test to look good, but at idle it had trouble seating. my exhaust sounded like put-put-put-put-put-PHUTT
