Interesting. I didn't sew nearly as much as you when I had the tubular belts. They'd work for a short time then stretch. I'd tighten them up, same thing. I also found I'd have to run them tighter than normal to not slip in the first place. I don't know the why of it.
I did try an experiment, I put a rubber band in the belt grove of one of the machines. That helped the top, but then the big wheel slipped. I gave up then.
I suppose it's possible the belt surface on the treadle and machines I've used is much slicker than yours. It is a viable explanation.
As for the clunking on the pitman arm I've got a couple with metal arms that do that too. Eventually I get around to cleaning and oiling them.
I have a Chinese replacement pitman arm assemble I got from Australia that actually has a spring loaded plunger in it that prevents the clunking. I like it. But I'm not gonna buy any more unless I find them in the US. The postage is too steep.
Joe