We were out this weekend on our '86 Sunsport and I'm concerned because in neutral and when moving at near idle speed, the temp crept way up to almost 220 degrees. Once we started moving, it quickly came down and stayed down. This kept happening each time we would come to rest. Does this sound like an impeller issue? It's not a weedy lake, so I don't think the intake was getting plugged. Any ideas would be great.