Does it feel like the car is losing power during its shuddering? A catalytic converter may not get plugged instantly. You have to experience gradual loss of power over time, before the catalytic converter is plugged.

Also, 150k miles are not too many for a plugged cat. converter, unless the gas that has been used over time was not of high quality (e.g., top tier gasoline). Other complicating factors are also the maintenance of the car over time and the way it has been driven.

Also, there is a rather simple check to do and see the condition of your cat. converter. There is a way of probing the T before and mainly after the converter and that could give a rather good ballpark idea of the cat. converter's condition. About 9 years ago, one of the dealers I no longer go to diagnosed that among other things my car's cat. converter (an 850 turbo sedan 1995) was shot! After I thanked them, I went to an indie who did the kind of test mentioned above (and I can tell you as a chemical engineer that it really makes good sense and it is effective) and he told me that our 850's converter was fine. 9 years later (and 23 years, 215+k miles since I bought the car) my converter is fine, perhaps with a slight loss of power I feel when I really accelerate hard for quiet a bit and many times in a row!