999S Error codes 10.1 & 11.1

Hi there,
2003 999S 27K+ miles, FDSH, good battery, properly charged, tank half-full of Shell V-Power (don't use anything else, except in emergency), spark plugs & fuel filter replaced (i.e., invoiced and paid for) during service last year (approx. 2500 mi ago), no sign of trouble before this morning.
On the way in to work this morning, the engine died briefly on me but picked up and carried on almost immediately. EOBD light came on, but went out. Carried on with fingers crossed and about five minutes later it happened again, just as I was trying to overtake a big lorry!
Picked up again OK, so I carried on to work. Didn't have handbook with me so found out how to read error codes and went out at lunchtime to see if any faults were logged - nothing!
On the way home, just beginning to think it had all been bike having a funny five minutes, when the engine cut out. This time it died and I had to stop by the side of the road. EOBD light definitely on this time, so read error codes and this time found a dirty great big "EFI" flashing at me, followed by "10.1" alternating with "11.1". Engine restarted after about a minute or two (EOBD light came on and then went out, as per normal) and then carried on home as if nothing had happened.
Got home, tried to read error codes and nothing again - as far as the engine management system was concerned, everything is fine!
Does anyone have any idea if this short-term attention-span is normal - or is it a symptom of more fundamental problems in the ECU (which was replaced under warranty when the bike was much younger)?
As to the error codes - at face value they mean I've got faulty power supplies on both ignition coils (which seems unlikely, if typical of my luck at times!). I'm fairly certain both coils have been replaced once already in the bike's lifetime (I'll have to check through the pile of paperwork I got when I bought the bike). Does this sound likely, or is it more likely that I've got an earth problem somewhere? The bike has had a fair amount of work done on various connections in the electrical system during its life, but nothing recently (i.e., in the last three years, at least). Doesn't see much use in the rain unless I get caught in a downpour while I'm out - but again, that hasn't happened recently. I washed it a fortnight ago and have ridden it since without noticing any stutters/stumbles or other glitches.
Any ideas/suggestions welcomed, particularly any ideas as to which earth connection to look at first. I did have other plans for this weekend . . .
Regards