- i had this problem on different occasions different components causing similar issue -

1) 1st problem radiator was too small (not same car but ford) and the flow wasnt enough for full throttle -
2) expansion cap: which reservoir are you using? OE or aftermarket - major leaks caused al sorts of pressure issues for me especially spillage from cap - i never got to bottom of issue as the problem has gone away
3) My heater matrix leaking was causing the fans to come one whilst driving at any speed - never assume brand new parts are perfect -
4) water pump housing leak caused the worst problem with temps rising like the radiator not flowing...

this was my experience my cooling is now fine although rs mk1 focus runs very hot lol

let the car idle and feel around which pipes are getting hot and which are not ( wait till fans come on IF they do that is)

IVE JUST REMEMBERED SOMETHING: My heater matrix pipes were plumbed the wrong way round so check that too -ah this is what caused the expansion cap spillage/ overflowing issue