bad cat means different things. it could be that the rear o2 sensors
are not seeing the cats cleaning out the hydrocarbons up. it could also mean that the cars are plugged up. You could check for back pressure and see if the exhaust is plugged up. If an engine misfires it sends raw fuel into the cat. that's when the check engine comes on and is flashing on and off as you drive. The flashing check engine light is telling you that you are damaging your cat if you continue to drive. Raw fuel in the cat will make the cat run too hot. It is supposed to burn off the unburned fuel from your engine. Raw fuel is too much to burn off so it will overheat and start to melt and then break up into pieces. When it melts of breaks up it plugs off the outlet for the engine. It can cause all sorts of problems. Eventually it can stop the engine from running altogether. The engine becomes an air compressor.
but first look at what is happening when it is missing. Do you have a misfire counter on your scanner? do you have any codes in the P0300-p0308 range? when the engine doesn't start have you checked for spark, fuel pressure and injector pulse to see what is not working?