Back on track
Join Date: Sep 2009
Location: United Kingdom
County: Greater Manchester
AFAIK the system is self bleeding. All you need to do is turn the steering from lock to lock a few times, this should get rid of any air in the system, then top up the fluid. I assume you have checked the steering fluid level?
Another thing that crosses my mind is the aux belt or tensioner. I am only guessing but when it gets warm, it will expand slightly and become looser around the pulleys. The tensioner should tighten to remove this slack, but if either of them is too worn, it may not be able to do this and so the belt will slip. You would usually expect to hear a squeal from the belt or the alternator charge light to come on the dash but this isn't always the case.
Hope it turns out to be something simple (ie cheap). Good luck.