Try to go straight, it goes more left or right, tighten cables it still does it. Fishing partner has also mk70 it does same thing. It seemed both motors were ok when new but as time went on they loosened up.
I would go with the 80 pound thrust, it won't have to work has hard as a 70 plus the extra 10 pounds will help in windy conditions and fishing heavy weeds. As far as steering goes they are all the same. If you want to be able to set your trolling motor on a straight course go with a Terrova with the I-pilot, this is a great trolling motor for open water, the spot lock is amazing feature that will hold you on you on the spot, great for jigging Small mouth or Walleye and that school of Spring Crappie. But if you are a Bass guy and are in the weeds and pads a lot, they can be a pain in the butt to clear off the weeds, as you have to bend over to bring the motor up where as the cable drive you can pull the lift handle and shake it up and down while your on the foot control to clear the weeds..... much fast and easier.
I actually find weeds easier to clear with the Terrova of weeds than a scissor mount foot control. 75% of the time, just spinning the motor 180, waiting a couple seconds, then hitting it in full speed for a couple seconds clears it. The resto fthe time, I just stand up, lift the motor straight up (don't release the latch, just lift the head of the motor straight up), and hit the power while it's mostly out of the water does it. It's very rare to have to do more than that to clear it.