Hi, hoping someone can offer some advice please

Have a 2015 1.0 Ecoboost Focus. A few times recently when pushing hard it has given the reduced power to lower engine temp warning, this has lasted about 10 seconds then returned to normal

Also noticed it was loosing coolant slowly, found signs of a leak around the plastic thermostat housing, so replaced that unit

This weekend, going along an A road at about 70, only 4 miles from home the temp gauge suddenly went from normal to max, with the stop warning coming on (first time this has happened) pulled over but could see no sign of overheating, car fans were not hot either

Drove back in limp and about half mile later the temp dropped right back and limp mode ended. One the 4 miles back it must have gone up to max about another 5 times, sometimes it went right up, alarm sounded, went into limp but dropped back within a few seconds, other times it just stayed in limp for minutes

Finally when I got to a hill it went into limp and I was stuck (cannot even pull away in limp mode on a hill) waited ten minutes the drove home ok

Thought it may have been a dodgy new thermostat, so removed that, also replaced expansion tank and cap, flushed cooling system, let it run for a while on driveway, did notice that although it was at normal temp, there was no heat in the heaters, but if i held the revs around 2.5k they would get fully hot in about 30seconds to a minute, let the revs drop again and the heat goes (no change on temp gauge though)

Drove it again today and exact same happened, about 4 miles from home temp suddenly jumped to max, alarm sounded to stop, limp mode, managed to struggle on for about a mile then temp suddenly dropped to normal and full power back

On way home temp went up to just below max, no alarm, no limp mode, then fell back to normal, few minutes later back to max and limp but only for around 10 seconds

Any idea what on earth is going on with it??