I have a 1988 Mach 1 Spoiler. It's about an 18ft boat with a 1987 Johnson 140 hp VRO outboard. Last weekend I had it out on the water for two consecutive days and as the weekend went on, The engine sounded like it was misfiring, more commonly at higher speeds. eventually it became more frequent at all speeds. The misfiring resembled almost like I was hitting something with the prop like a log or something (obviously I wasn't as I was in 60ft water), where the whole outboard would shake. Any ideas? Thanks!