2004 1.6 life Zafira Power steering problem?

The power steering on my Zafira 2004 1.6 Life keeps stopping working. The steering becomes very heavy but when the weather is very cold it starts working again. Stop at traffic lights it warms up and stops working. I've checked the output from the alternator and it is giving 14V. I've also checked the fluid level and it is OK but the colour is amber rather than red or green(quoted colours from elsewhere). Could it be that the fluid is too runny but becomes thicker when it is cold? or does the output from the alternator change with temperature? or is the pump due for replacement?