should be mech fuel injection, so worth checking fuel filter, clean injectors, check injection pump, etc.. its not that complex of an engine; its like a 1/2way house between carbs and EFI..
my first though from what you have said would be spark/electrical; if its spluttering and dying it could be fuel, but if its missing its more likely spark.. Check your dizzy, cap, rotor arm first, sound like they might be on their way out, and then check out the coil and plugs..
if everything looks ok, and you are tracing a good spark all the way through the system from coil to sparkplug, dry and clean everything, and give it some of that silicone spray; its very damp out at the moment, and could be something as simple as moisture knackering your spark.. failing that, dont forget the LV side of the system; its possible you are getting a poor trigger or charge to the coil if you have a bad earth or break in a wire, although I would expect that to show as an intermittent spark (ie sometimes nothing at all) rather than an occasional missfire..
good luck, and if you need any haynes manual stuff let me know; I think I have a petrol and diesel one kicking about somewhere, I can always scan you a few pages
ben
