If you are losing power only after a long or highway drive - is your catalytic converter partially blocked and getting worse when it heats up?

I saw this in a Toyota belonging to my mother. She never drives long enough to warm the engine thoroughly and it did not show any symptoms for her. I took it out for a long highway run (>1 hour) and it lost power. Her shop drove it a while and diagnosed the problem by putting a temp probe on the cat - replacing the cat cured the problem in her case. It was suggested the original cat was falling apart internally, though we didn't look inside.

Just a thought...