Steering wheel jerks back and forth in a turn?

[musclecarman13]

First of all, this is my first post in this forum-there seems to be a lot of knowledge and experience to learn from and I look forward to your advice....

So this is what's going on-I just became the proud owner of a 2008 F250 6.4 CC SB 4x4 with 78K on it and so far I love it-but there seems to be an issue that of course I didn't notice on test drive. (Funny how that happens) SO what happens is it turns fine sitting still and with or without applying throttle-everything operates smooth...But if I am in the middle of a turn and holding a steady steering angle it's like the steering wheel jerks back and forth at least an inch or two. When I apply a little more effort I can easily get it to straighten back up and it will track straight going down the road with no drift or anything.

So far all I have done is check the PS fluid which was a little low so I topped it off but that didn't seem to change anything. My stepfather has an 05 F350 Dually CC LB 4x4 and has never had a problem and neither has anyone else I have talked to. Please tell me it's not just my truck!

Thanks
Phillip