I don't need anyone apologizing for me. I can do it myself. I reread and see where the OP stated that the problem existed even with the 265's. So, in light of my mistake, I apologize.

 

what kind of shape are your cats and mufflers in?

it sounds almost like back-pressure relieving itself when it reaches enough strength to do so.. and it would certainly run rich before that point...

you say it started half-way... did it set in from the clear blue sky? as in, it seemed fine and holding a steady RPM and all the sudden started? did it start after a bump in the road? does it do it if you ease into the throttle or just when you jump on it?

the folks already making suggestions are much smarter at this than I am... I'm just tossing other things I haven't seen yet.. If there is a big chunk sitting in either your muffler or your cat(s), that could very well be causing these kinds of issues, as opposed to the thing just disintegrating..

grab up a rubber mallet in one hand, and cup the cat/muffler (s) in the other, and gently tap those bastardos, and listen/feel for movement inside them..

It stands to reason that accelerating w/o load wouldn't generate enough pressure quickly enough to dislodge a chunk and put it square in the exhausts path- while accelerating under load (more gas quicker) can generate enough force to push a chunk in the way... and once the engine starts to produce some serious pressure (3k rpm+) it clears a pathway and runs... how did you put it? like a raped ape? A good bump in the road could also dislodge the thing and obstruct flow while underway..

yup.. i'd tap that rascal with a mallet and feel for movement within..