Steering problem

[ben@speedbarn]

I have been having a problem with my steering on my 76 GMC k2500. I got the truck in pretty bad shape about a year ago. Put about 200 miles on it and didn't notice anything wrong with the steering. Since then I have fixed most of the problems with the truck and replaced all the springs and shocks including a 4" lift. Now when I drive down the road it will be going straight and then randomly veer into the left lane(with the steering wheel staying straight). This is going about 10-15mph. When I turn it back to the right it takes a bit before it will feel like it catches...then it goes too far to the right. All the ball joints and everything look fine. When sitting still both tires start turning at the same time as the steering wheel in both directions and there doesn't seem to be any slop. I also tried tightening the adjustment on the steering box but that didn't help at all.

My only guess is that there is something loose inside the gear box but i wanted to get some other opinions before I drop the $150.

Thanks.