Just bought a boat w/89 Force 50hp outboard. The test drive was fine, as was the first 30 minute ride I took from my place. "no previous problems" according to previous owner. The problem started the next trip. About 20 minutes in, the motor started "klunking/kicking". You can hear/feel the klunk throughout the boat. By the time I got back to shore, it was occuring about every 15 - 30 seconds. At slow speed, it does not seem to klunk. I then drained/refilled the lower unit oil. Old oil was a bit dirty...not bad though...didn't see/feel any solid bits in oil.any thoughts?