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.

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1986 300E 5-Speed 240k mi.