Misfire

My Jimny trialler seems to have developed an intermittent misfire. I loaded it on the trailer for a trial yesterday and it was running fine, as usual. Unloaded at the trial and agin running fine. I left it ticking over for about 20 mins to warm up a bit before the first section. Driving over it was misfiring a bit and then would only tick over on 3 cylinders. If I reved it to about 2000 rpm it cleared but when driving it was still missing at low revs. It did this all day! I loaded it up and when I got home it was running fine again, no misfire, and the same again today, when the engine was cold.
Its a 2000 G13BB with very low mileage, and had new plugs about a year ago, but hasn't really been used much in that time. Part of the problem maybe that it never really gets hot, and does a lot of stop/start driving. Its not on the road so will never be run hot.
The engine light is not on although of course that doesn't mean it hasn't a fault code. I plugged my OBD11 reader in but it couldn't connect, as expected really. It had the cat removed a little while ago and has been fine since then.
So any ideas as to what to change/look at very much appreciated. I suppose another new set of plugs might help, but I'm not totally convinced as its ok now.

Cam sensor.

Thanks J999pre - I had wondered about that, and of course the spare one I have is for a M13 engine! I will try one and see what happens.