Help classic impreza intermittent misfire

[Wattsym12]

Classic impreza intermittent misfire originally started on drivers side could hear the engine go tick tick and every time it did the car would misfire I originally thought it was a sticky valve or something like that but today had the intermittent misfire and the tick tick noise was from passengers side this time confused as to why it would swap sides ? Any ideas greatly appreciated

[ingrowtn]

I'm sure others will know more than me - but I just fixed a misfire on mine with a set of new HT leads.