Merc 120 runs good, then not so much

[Bdt 1967]

I have a 1980 Merc 120, it starts right up and will idle all day long, this past weekend, however, I was cruising across the lake at relatively high speed, when I chopped the throttle the boat idle rough, coughed once, and just sounded bad (Like it was missing bad), I put the boat in neutral, brought the rpms up to 2k then back down and she was back to normal, smooth as ever. Decided not to pay much attention to it, cruised to the place where we beach at, slowed down and again she was rough, popped it into neutral, revved it to 2k, brought it back down, and no luck, still rough. Idled over to the beach, and slowly she was getting smoother and smoother with every passing second. By the time we reached the beach the old boat was smooth as ever again. This happened a total of 3 times this past weekend. Not sure what could be causing it, appears to be cooling just fine. Not sure what could be causing this. Anybody ever heard of this. The boat has good gas in it, good plugs, not sure on the condition of the wires and points. Any suggestions?