Sound similar to a problem FIAT had with their electric steering pumps (or its control, I can't recall which). Can be caused by overheating of the pump, but not I suspect at this time of year. I seem to recall that FIAT would only replace the whole unit (yes, expensive) but that independant speicalist could repair them. Think you would need to get it diagonosed properly by a Vauxhall dealer.

Well good luck, perhaps will turn out to be just a sensor, but I'd prepare yourself!

You might find this website helpful if the worse comes to the worst. I haven't used them myself, but others here may have.