The miscreant: my venerable 1990 FJ1200 3CV(approx 170k miles on the clock but full strip-down and rebuild at 75k miles following chain breakage cracking crankcase).

All was running ostensibly smoothly following oil change, carb balancing etc. The problem: on a wet ride it suddenly went permanently onto 3 cylinders. Dried it out, carried out grounded plug test on all 4 pots and no.2 was found to have a weak and erratic spark. Replaced all 4 plugs. Still no good. Nipped a few mm off the end of no.2's HT lead to get a better contact. Some slight improvement. Replaced no.2's plug cap. Again, possibly some slight improvement but still misfiring drastically. Swapped over leads of no.2 and no.3 (they fire together). No apparent difference but hard to tell which was misbehaving. Put the problem in the "too hard" tray and used other bike for several weeks.

Puncture in other bike so forced to use FJ12 again for a few days. Surprisingly it started up immediately so I've been riding it into London for the past couple of days. The present symptoms are that on drawing away it fires on three at best and often only on two but, mysteriously, if I grit my teeth and thrash it hard it eventually reverts to firing on all four but when the throttle is eased it's back to 3 or 2. Under load on the motorway it will pull well to 6500 rpm or beyond and is running on all four, although not as smoothly as usual and down on power. The fuel economy, needless to say is very poor.

I am mystified. Sometimes it seems like a fuelling problem but I don't think it can be because the behaviour doesn't seem consistent with that. At other times it seems like an electrical problem but it's odd that it sometimes affects only one cylinder and sometimes two. Since 2 and 3 have the same spark source it presumably cannot be the igniter box. Could it be dodgy HT leads at this age/mileage?

Any wisdom or suggestions would be much appreciated (but not "bin it", please - I'm very attached to the old girl having had her from new) .
Neil.