06 Pacifica, 3.5, AWD, 54k miles with intermittant PS problem

When cold the steering acts as if the engine had cut out, very hard to turn, does not last long and is gone after a minute of warm up. Does *not* happen all the time. PS fluid level is good.

Could it be:
1) belt (does not look worn or cracked)
2) belt tensioner
3) pump
4) rack
5) something else?

Thanks