2001 1.6 Overheating problem.

Hi, I'm just wondering if anyone could give me a bit of help, or if anyone has had (or is having) the same problem as I am having..

i have a 2001 Ford Focus it is a 1.6 Zetec, and it is a good runner most of the time.

Now for the relentless problem I am having.. When driving under 40mph the car runs really well without a problem, but once i hit a motorway or have to go over 50mph the temperature gauge will shoot straight up into the red until i slow down. When i first crossed this problem my first thought was that the thermostat was dud, so I had it changed, I also changed the water pump after reading a few posts from other websites and I changed the coolant and my uncle compression tested the car and it ran well for around 3 weeks, and now the problem has started again. I was told to change the coolant temperature sensor I have ordered the sensor and have to pick it up on Monday morning, But something is telling me that this is not going to fix the problem..
So I am here to ask if anyone has ever had this problem and if they have would they tell me how they fixed it.

Thanks :)