When my truck – it’s an old truck – does that teeter tottering thing its usually b/c the shocks are worn out. I notice it most on bumpy roads around 20-25 mph. It feels like I’m riding a horse, what with all the bouncing around. Replacement shocks fix it straight away. So in addition to the good ideas posted above, you may need some new struts on your Matrix.
It’s possible the rotor warping is unrelated. Next time you have the rotors replaced, replace the pads too. For some reason I tend to have best luck with replacement pads that are priced midway between the most expensive and the least. And while they do that job, ask them to double check the calipers are sliding well on their slides, and the pistons are moving freely. They’d normally do that anyway, but no harm asking them to make sure. There’s one other thing that is a common cause for rotors to warp prematurely, over tightening the lug-nuts, or not sequencing the lug-nut tightening pattern correctly. On my Corolla, which has 4 lug nuts, I first snug all the lug-nuts up by hand until the wheel is evenly but lightly pressed against the hub. Making sure the bolts are properly centered in the holes in the wheel. Then I tighten alternate nuts spaced 180 degrees apart to 30%, then the same for the other pair. Then I do it again the same way, only to 60%. Then a final tightening to 100% of the manufacturer’s recommended torque value. I can’t claim that’s the reason, but I’ve never had a warped rotor.