Have you tried unpluging a spark lead one at a time to see if its on the same pot or moving about? if its on one pot then i guess your looking at dizzy/lead/plug....if it moves i guess its dizzy or ignition modules...

just a thought for you but i would start with the less $ solution approach...

bassically look at what it could be, make a list, then price replacements, then look at what is either due for replacement (or stuff that is cheap and could be due)...

so for instance if you havent done plugs they are cheap and probably due (or wouldnt hurt) so do them, followed by other componants in lowest to highest price order...

im perhaps thinking coil....they arent mega money and when they fail they can fail weird....but its only cos i got caught once myself...

my 'its free so do it' advice would be remove all ground straps, clean and firmly re-attach...afterall they are the 'other side' of the spark plug loop as it were....and are subject to corrosion in 'interesting' ways!

but then again these are just ramblings...