micra running probs?

hi i have a 94 nissan micra 1300 manual ,it broke down a few days ago, the AA patrolman said it was a blocked catylitic converter , which ive had changed ,but its still the same.its ok when its cold ,as soon as it warms up tho ? it starts misfiring .the engine revs up and down,on its own ,then cuts out totally starts ok a gain but revs wont pick up ? will only tick over, a few fitters have looked at it but failed , ive also fitted a new fuel filter anyone got any ideas regards clive