Hi,
I have a 04 plate rx8 and the other week it died and took ages to eventually restart but had very little power and ran very lumpy then there was smoke starting to come inside from the exhaust been red hot, I have since replaced the cat and both O2 sensors and the plugs and still exactly the same!!
Any help/info would be excellent.

 

Did you also change the coils? How many miles on your car? Is your compression good?

 

Since you had to replace the cat it's possible the last one broke up and clogged the muffler. Did you drop the rest of the exhaust to shake out any debris? Is the new cat you put on a new or used cat? And is it an OEM or a generic replacement one?

Also like the last poster asked did you replace the coils when you replaced the plugs? The coils on these cars have a short lifespan. If it is coil related and you don't replace them it could quickly destroy the new cat you just put on the car.