Sudden temperature spike - confused!

I carried out a coolant change last week on my TF 160, as I had been having an issue with a lot of back pressure finding its way into the expansion tank. I also replaced the expansion tank cap. I bled it from all 3 points, all seemed fine.



I took it for a test drive and everything seemed absolutely fine, until coming up the fairly steep hill back to my house, when the temperature gauge started rising. In the 30 seconds it took to get back to the house it rose several notches. I let it cool down, then checked the coolant level, which seemed fine.



The same thing happened yesterday when my girlfriend drove the car to work, it was fine on the way there and back, until coming up the hill to our house, when the needle started rising again!



The car is sat on a level driveway, and I have just started it up and let it run for 10 minutes. Engine came up to temperature fine, sounded absolutely fine, I couldn't detect any leaks and the coolant level has remained constant.



Any ideas what could cause such a spike in temperature? Before I changed the coolant, the car hadn't been driven for a month, and when I let it idle just now, the rad fan didn't come on, but I don't know whether the engine would get hot enough to require the fan if it's just sat idling.



Thank-you in advance for any ideas as to what might be causing it.

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