1997 GMC C1500 Steering problem

Steering problem
1997 GMC C1500 V6 Four Wheel Drive Manual 202000 miles

My steering has been acting up. It does not happen all the time, only when it wants to. I'll be driving down the road making a gradual turn in the road and the steering will jump to the right or left depending on which direction I am steering. It feels like it is spring loaded. Also the steering feels mushy at times when it acts up and will feel like I don't have power steering. My mechanic changed the gear box twice and still having a problem. He suggested changing the power steering pump. Just wanted to know what your thoughts are on this. Also I remember having the steering acting up being hard to steer and going to the store. I turned off my truck, went shopping, came back turned on the truck and the steering was back to normal. It's driving me crazy not knowing what is wrong and don't want to get ripped off. No body seems to know what the problem is, please help.