"Dishing" on a horizontal saw

It has to be the actual hinge mechanism being worn, seems to me. If it were my problem to diagnose, I'd start with a square piece and catch it in the vise so the square end is just against the blade, then carefully and slowly raise and lower the blade to see if the blade wanders without any load, saw turned off. If it does, then for sure it's the hinge ears. If not, then it may be some strange blade issue, and I think the second thing I'd do is to first carefully check the guide adjustment and blade tension and see if that helped, and failing that I think I'd try a new blade.
GWE

Too much down pressure for the blade speed in alum? Guides worn? The back guide that you can move in and out for different dia work not as close as could be?

Try a different blade.
Rich

--I've learned that it's possible for a blade to be *too* tight. You might want to ease off and see if it gets any easier. Also for aluminum cutting, particularly thicker stuff, use *very* high speeds; i.e. similar to what you have on a woodcutting bandsaw, with a blade of at the most 5 teeth per inch; 4 is better.

I had a Kalamazoo, nice saws. Two things cause this kind of problem. The first is the wrong blade tension. Too loose and the blade sags, too tight and it stretches more at the back (soft area) than at the teeth (hard area). Take the blade off and lay it on the ground, if it wants to curl up you had it too tight. The second thing is weight on the blade. Too little weight and the teeth rub instead of cutting, so you get poor blade life. Too much and the blade twists to one side. One side of the teeth then do more work, get dull and the problem gets much worse. Talk to the manufacturer about the proper weight. By the way, just adjusting the hydraulic don feed valve will NOT correct this problem. You either need to adjust a counterweight or a counter balance spring.
Sawing with too fine a blade is also bad, especially on aluminum. Aluminum cuts easily and the chip rolls up in the gullet between teeth. One the teeth pack solid you get a slow crooked cut and the cut face looks smeared. Teeth should be as coarse as possible, while always having at least two full teeth in contact at all times. Too coarse and you risk breaking teeth off or chattering badly. Most people err on the side of too fine, and would be better off being a little too coarse.
Vari-pitch blades are wonderful. They allow cutting a broader range of material thickness and greatly reduce chatter on hollow sections.
Hope this helps.
Gary H. Lucas