Hi All, I have been trying to get a diagnosis on an issue for months. I took my 2015 to a mechanic and they can't find the problem, but think it's a headgasket crack and I am guessing a headgasket repair is not worth the value. Especially if it's only a maybe.  My Focus has been fine for city driving but when driving higher speeds 70mph or going uphill on country roads, the temp will fly up, I lose revs, but it comes back down to reg temp within a couple of miles and away I go again. It might be fine for another 100 miles, or sometimes not many at all. Cooling fans will stay going for maybe 5-10 minutes after shutting engine off. It's been like this for 12 months now. I also find myself topping up coolant every month, which resolves it for a couple of weeks at a time.

Not clear if it loses coolant due to a leak or due to it being pushed out of the pressure cap when it overheats. In the old days these symptoms would suggest a partially blocked radiator core which entailed backwards flushing the radiator to get silt and debris out.  I do not know if this is ever a problem these days. Your car is not terribly old. But I also wonder if thermostat has been checked/ replaced or you have tried it with thermostat removed.   I do not know if this has a good old wax thermostat or something stupidly electronic.