323is loss engine power! speed<20m

Hey guys, my 98 323is suddenly loss engine power, idle looks fine, but anything above RPM 3000 the engine will shake (i can feel the whole car is shaking as well) and then the engine turns off by itself.
Car is derivable but regardless how hard you push the gas pad the speed and RPM won't goes up, the speed won't exceed 20m the RPM won't exceed 3.
I went to BMW and they told me my catalytic converter need to be changed!!
I just changed a whole catalytic converter last year and then I need to change again?!
They told me the middle box on catalytic converter is blocked and the air won't be able to get out so that's why I'm losing my power..
Not sure what to do, anyway to clean the old catalytic converter?

If the catalytic converter was replaced last year, I HIGHLY doubt that's the issue. A word of advice: I wouldn't recommend taking your car to BMW for service. It may seem counter-intuitive to not recommend taking your BMW to the dealer for service, but the reality is that A) dealers are notorious crooks, and B) they hardly every work on E36 BMWs anymore. Their service technicians know far less about our cars than they do the newer cars. If you want knowledgeable service for an older BMW, you need to go to an independent shop which specializes in BMWs, or at least European cars. They see the older BMWs much more frequently, and in my view, know how to fix them much more effectively than the BMW dealer does.

As far as what is causing your power loss, it could be a couple of things. Fouled spark plugs, vacuum leaks, etc. can all cause the symptoms you are describing.

These weird symptoms don't make much sense. Could you give us more information? Is it an automatic stuck in limp mode, maybe?