I had an 77 Chevy truck that had a clogged, and was sluggish from just off idle. It litterly went clogged overnight for some reason.
Also it could be your distributer not advancing causing the same issue. A friend of mine got bit on this one. He replaced his cat, but it was still his distributer. A $20 fix vs the $250 he spent.
If it were my truck I would get a timing light out, and bring up the rpm to see if the motor timing advance's.
On an aba which I have too I am not sure. I still need my bently manual.