Neil, out of the blue, check sparkplug wire connection. One may not have been put back on correctly. I had one vibrate lose when under power and scared the crap out of me not knowing what could be wrong. My brother in law was with me and it use to be his boat and he said it happen to him once. He said check the plug connection. So, I pulled the cowling and pushed all of them in hard and it fired right up and never bothered me again. I put small zip ties around the plug connections. Not a problem since. Hope you get her going!