The clogged cats in my travels have not caused intermittent problems.
They would consistently "hit the wall" at a certain speed, and positively slog under load, like when climbing a hill.
Just my results.
I'm sure there must be some sort of vacuum pressure test that can be performed for a positive diag, though I do not know the specifics.
Best of luck.
1986 300E 5-Speed 240k mi.