Power steering issue - 1999 Toyota Camry LE 4 cyl, 53K miles

My daughter has a 1999 Toyota Camry LE, 4 cyl with 53K miles. Twice in the past three months the power steering on her car has cut out for a few seconds while she has been driving at low speeds. We have had it in the shop both times and they have no idea what the problem is. The last response we got was, since we can't replicate it, we will just have to wait until it gets bad enough that we can. The fluid and the belt appear to be fine. The only other suggestion that we've gotten was to replace the pump and the rack.

She has only been driving for a couple of years, so this problem makes her very nervous.

Thanks.