I think you have a bad tire(s). Some tires go bad by the internal belt breaking down which can cause a slight side to side action at slow speeds. I don’t think a tire going bad in this manner would have any impact on the rotor problem, most likely the two issues are not related.
Wrapped rotors are most commonly caused by excess heat. Perhaps you have a dragging caliper overheating the brake(s). Cheap rotors use inferior materials and are more prone to warping. Make sure the next set of rotors are high quality parts, if you use the same shop ask them to use a different brand of rotor.
Try having the wheels spun on a “road force balancing machine”. That spins the wheels while applying a simulated road force by pressing a spinning drum against the tread. It can detect internal tire problems that a regular spin balancer cannot.
Also try having your car aligned. Out-of-alignment conditions can cause wandering.
I like the out-of-round tire thought. That’s just for the teeter totter problem.
As for replacing brake rotors every 6 months, I go for a dragging brake caused by a stuck caliper of bad master cylinder. This assumes, as @oldtimer 11 suggests, that you are not resting your foot on the pedal or you are not a go/brake/go/brake kind of driver.
If your mechanic has not identified either situation as a problem, it is time to change mechanics.
My first thought was a bad tire which can exhibit those symptoms.
Given the comment about the regular replacement of the brakes I might suggest that a caliper could be sticking on its slide or the piston in the caliper is not releasing fully due to aged piston seals.
That can cause premature wear of brakes, warping of rotors, and also cause a lurching feeling at slow speed.
The guy doing the brakes should be servicing the slides as part of the brake job and as to piston seals that can usually be detected by the degree of difficulty involved in retracting the pistons into their respective bores during brake pad replacement.
I’m not saying the guy is guilty of neglect; only pointing out a supposition on my part. Hope that helps.
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.