I believe high speed wobbles on a downhill come from the weight distribution shifting forward over weighting the front end, but I've never experienced it as I was taught to shift my weight back on downhills...

I love 531 frames.

But to OP: I'd guess any 80s-90s medium priced bike from Japan, Italy, England or America would fit your needs if in good condition and reasonably maintained, or if not, if you want to do some work yourself.