power steering that works sometimes?

I've got a '97 XLT V6 w/ 87k. I noticed a problem this fall; after letting the truck sit for a couple of months the power steering got intermittent - it gets easy and hard as you turn the wheel. After driving it a mile or so, the steering felt better, but still acted up a couple times breifly. I then discovered that when Jiffy Lube says "Chassis - Lubricated X" they just mean the oil that drips from the filter - the ball joints were bone dry. They're lubed now. I don't know if they're original. I did brakes not long ago and found the joints to be tight ("If it ain't broke..."). Pump doesnt sound abnormal. Does it seem like its a ball joint problem, a steering fluid change problem, or a real steering pump/rack problem? The steering acted up again today after sitting about 3-weeks (and I'm in NY where its REAL cold). Thanks in advance - Keith