Sounds like a PAS issue. I would start with bleeding the PAS system, however I would expect it to be the PAS pump.
Worth bleeding it just in case though - however it would normally bleed itself under use.
Jack both front wheels up off the floor. Take the PAS res cap off. Check the fluid before hand.
Turn the wheels from full lock to full lock about 10 times, and check for bubbles in the PAS reservoir.
Shouldn't take more than half an hour at the VERY most - so worth a go.
If not, I;d be looking at the PAS pump.