It could be the ps box, not the pump. The steering flexes a shaft which controls the hydraulic flow, and if a seal is leaking, it can direct flow to steer to one side. Usually this will be affected by engine speed fairly consistently as the flow/pressure goes up and down.
However, I would take a look at your left axleshaft universal joint as it is constantly rotating. It could be binding up and causing a jerk of the wheel.
I would also take a look at all the balljoints/tierods etc. And maybe even consider the front diff if it has had any interesting use lately. Peices of metal could be going through the spider gears which would not lock the wheel, but would lock the carrier and the inertia would breifly jerk the wheel.
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