Why does my VW Golf judder and loose power?

[Caroline6]

I own a VW Golf driver - I have it regularly serviced but still when travelling at around 70mph it will sometimes start to judder and then loose power. It usually does this when there is an upward gradient in the road, although it doesn't need to be much, then becomes fine once going downhill. It is quite random, sometimes the car is fine, then other times it will start, I change down a gear and rev it up a bit and this sometimes seems to stop it!