Overheating Problems

Hi there, I am Jatocmo

I'm new to this lark so hope his sounds right.

I have an Octavia 1.9 DTI 06 reg. Up to last week the temp gauge has registered 90.without faultering, however last week it started creeping up the scale as my speed increased and as I slowed down the temp reading began to drop again. I am not losing any coolant and the expansion bottle levels are as normal. A technician friend who doesn't work on Skodas normally has suggested that similar to a fault on a VW Golf he wonders if it could be the impella in the waterpump has loosend up on the shaft and when I put my foot down it slips. I can keep the temp down for at least 10 miles FROM STARTING COLD IF i DON'T PUSH IT HARD AND NOT ABOVE 50 MPH.

Has anyone come across this before?