Random missfire

Hi all.

We recently had some starting trouble with my girlfriends k11 micra celebration 1L. The car worked one day and failed to start the next. After finding that the sparks were quite weak we replaced the HT leads and spark plugs. As the rotar arm and distributor cap looked a bit corroded we changed that too and the sparks were then all good. Sure enough the thing fired in to life, however ever since it has started again it has developed a missfire. If you leave the car idling it will bog down every so often at completely random intervals, and when driving it feels like the engine practically cuts out before carrying on. I've noticed that the timing chain seems to have also developed a bit of a rattle at the same time and I'm not sure if this is linked as surely if it was a timing issue it would misfire on every cycle. I guess with 68k on the clock it would be wise to change this but I would like to get to the bottom of the missfire before doing so.

I've also papercliped the car and it flashed 55 which as you know is "normal"

Any advice on what it could be would be great.

Thanks in advance for any help