1997 Chevy Suburban 97 Suburban Steering jumps during a sli

My 97 suburban has been starting to act up when steering. In a turn whether it is high or low speed, the steering wheel will pulse or jump a little in the direction i'm turning and the car turns a little more when it happens. It has been worsening and becoming a harder pulse as time has gone on and will almost jump the car into another lane when lane changing. It doesn't seem to happen durring a hard turn but on like an on/off ramp or changing lanes. I also have been experiencing durring excelleration the steering kind of shimy a little, it feels like the wheels are out of balance but I just had that done when they were rotated. I am concerned of course that it will get worse and at some point I won't be able to keep the car under control. I flushed out the power steering fluid multiple times which was black and replaced the fluid and added an additive to help condition the seals. But it is still happening. What could it be and how do I fix it.