bad steering problem

[72K10]

I let my truck sit for a few days. Drove it this morning and it jerked hard to the right every few seconds. Almost like it caught a groove in the road. When I would pull it back to the left, it would then jerk over to the left, again like it was in a groove going the other way. Almost got in several accidents.

the odd thing is that after it warmed up, (20 miles or so) the jerking went away, though it still pulled to the right at times. Just more smoothly, rather than a jerk. I've swapped the left and right front tires, but no effect on it. The steering has been doing this sort of thing for a while, but never this bad, and it always gets better after it warms up. Any ideas??? This is a 4x4 with power steering.

Thanks,

Chris