??
My '72 Bonneville has begun to diesel when accelerating. It began just when really pressing hard, but now does it on slow starts from a stop sign. I've put high octane in it, to no avail. It currently has 122K; does it sound like the timing chain's slipped?

It should like your carburetor. Needs replacement or rebuild the same one.

Many things can cause your problem. First check timing, distributer weights can be stuck in an advanced position causing your type of problem. Carbon buildup in combustion chambers. Considering the year of your car, I would try one of the lead substitutes made for older cars. It may not like unleaded gas.
Most likely problem is advanced timing, check that first.