2002 Sierra power steering issue

I bought this exteneded cab long box last Feb. and since then its been trouble free with one sudden exception. The power steering drags occasionally when turn left at basically parking lot speeds. I've checked belt tension, fluid level, and had the truck lubed, all seems fine. I talked to a local indy and a couple of techs. One asked if it occured when the A/C was on or off and mumbled about some pressure valve?
I've yet to take her into GM to check it out but was wondering what the Tech here though of the symptoms.
Thanks for any response,and
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