random misfire

[NISMO SER]

Hello all

I've been battling with a random cylinder misfired problem for quite a while
now but still couldn't fix it.  The check engine light would come on
randomly.  I pulled the code and it said "multiple random cylinder misfire".
  I did the tuneup,
clean the EGR system and checked for leak but I still can't fix the problem.

The car would run fine without the check engine light for a couple of days
then the engine light would come on again after a few days.  I also failed
emission test recently (nox failed).  The only thing that I didn't replace
is the catalytic converter.  Would a failed cat cause multiple random
cylinder misfire?

thanks in advance
David
98 SER