I've take the thing apart and gone through an adjustment and cleaning process. It looks like the miter lock has been overtightened and the clamping plate deformed.
I've removed the clamping plate and used a piece of bar steel as a temporary replacement. That seems to reduce the movement significantly.
I've ordered a replacement plate from Sears Parts Direct and will see if that solves the problem.
Re: tool quality - I bought this in 1998 for @$200. At the time, a mid-grade saw. For me, it was a good, available tradeoff between cost and my homeowner DIY tasks. I'll be more careful with the miter lock in the future.