Clogged cat?
 

Would a partially clogged cat cause an exhaust sputter?

Clogged cat
 

Yes, a clogged (or broken) cat could cause sputter. More often there's a very large reduction in high rpm power from a defective cat. Occasionally (in severe cases) there will be reduction in intake vacuum at high engine loads (accel or hill climbing).
Sometimes a broken cat's pieces will bounce around producing intermittent problems (or lack of problems). If yours are bad, take the opportunity to look thru the cat and compare to a new one. And knock some of the insides out and look how it's made. Imagine some of those pieces turned sideways so the tiny ducts are sideways to exhaust flow; they won't pass much exhaust.
Leaky valves also cause exhaust sputter. We used to hold a playing card against the tail pipe to check for smoothness and pop-back. Lots of popping usually meant poor valve closing.
Hope this helps

On my 87, the CAT broke up into pieces and came out the rear and got caught at the "Y" junction of the tailpipe. Couldn't get any high revs till we diagnosed the blockage.

That's kind of what I ran into. I'm running true duals so I took a pipe down and shook it and a couple of chunks came out.. did your sputter when it was messed up, or just restrict performance?

since its breaking up, go ahead and finish ramming all the stuff out of the main cat. Hopefully none of it made it into the mufflers :(

Time to take the cat off and inspect.

I'm guessing this is a 84-91 since you stated cat and not cats.

It's an '86 with true duals, it's the cat on the passenger side. I dropped the one pipe because some did make it to the muffler and i shook it and alot came out, maybe the size of a golf ball. I've been getting a sputter from one side after playing with different rocker adjustments and such so i'm thinking this might be the reason