I had a similar experience years ago on a 3 cylinder Johnson looper. I was young and naive in nautical mechanics at the time, the engine was purchased used from a friend and I hadn't run it probably 10 hrs. He said it was HOT. Well it was and did funny things as you are mentioning.
One day I was at WOT blasting across the lake and BAM.....just about buried my teeth in the windshield....the sucker locked up and didn't loosen up. Result was a .030 bore job with new pistons and all.....the stat was frozen shut and I don't remember if it had pee or not or if so, how it was plumbed and before the days of OT warning sensors.
Check your stats (at the top of each cylinder bank) and popoff. Opening temp is stamped on the bottom of the pellet and if I recall on my son's 150 they are 143F. Have to look on a parts list to see where your popoff (high rpm pressure relief) is located or ask Faztbullet on here. I think it's connected by hose from the top of the block in center rear down to the mid section in the rear center where the valve is located, thus dumping into the midsection ....speculation; not verified.
