"...or fuel brain...."

A 240 turbo? What "fuel brain?" The 240 turbo uses the K-jet "mechanical" FI system.

"...co-worker suggested that the cat is bad..."

I had a bad cat on my '82 245 turbo when I bought it (it's the reason I could buy it...).

The symptom was no power above idle. Your description of the symptom could be a bad cat, though it's a stretch.....

You'll find a threaded plug in the exhaust pipe 'tween the turbo and cat. Remove this plug (yes, I know it's louder than hell...) and see if the performance slightly improves. If so, then you might indeed have a plugged cat.

In my case, the feeling was that the engine was severely choked or constipated. I would not characterize the problem as "stumbling" or "sputtering."
--
Don Foster (near Cape Cod, MA)