I had a similar problem with my L20B, would randomly die sometimes, would run fine other times. Ended up stuck on the side of the road out in the bush. After changing dizzy and coil it started and ran fine. That was a few months ago, and its still going fine. Dizzy was a fairly new Bosch Scorcher unit... dont know what was wrong with it.

Make sure the points aren't closing up.

Even wilder stab!

if your missing the resistor, chances are that your coil may be overheating and the charge breaking down (sometimes its the simple things and replacing the coil and the ute running ok for 5 mins makes me think this may be the problem).
The coil is usually oil filled and once oil heats it breaks down a bit... this is my stab in the dark

Ok, a bit of background:
It doesn't have a choke, but when i first got it going it drove like a beauty. Middle of winter it would start up no worries and just go. Slowly, after a couple months it backfired through the carby once or twice when cold. Slowly it started backfiring more and more, until it became virtually impossible to drive when cold. I had to hold it at about 2000rpm for a minute or 2 before it would be warm enough to back out of the garage. Then it would still backfire heaps. 5 minutes after i took off it would still backfire while just cruising along 70km/h not accelerating.

I put up with this for a little while, then today I was driving home, and the engine just died. A slight splutter, and gone. I ended up pulling up right outside dads old mates place. Knocked on the door and he came out and gave me a hand. Tracked the problem back to the coil, replaced the coil with a spare that was in the old 200b parked out the back somewhere. All sorted, gave it a bit of a tune, and it ran like a dream again.

Tonight, the next time i drove it, drove for about 5 min, and it died again. Exactly the same thing. pushed it to the servo, and picked it up about an hour later. Tried starting it, and it started up no problems. Drove for a couple minutes and it died at the lights, and had to be towed home.

Its missing a resistor that is supposed to go next to the coil. I really don't know whats wrong with it, but since it started up after it died without touching it, i'm guessing its not the coil. I plan to get a hold of another coil, complete with resistor and what not, rewire the whole coil circuit (or as much as neccesary) and replace dizzy with an electronic one (have a points one now)

I'm also running a thermo fan and a fuel pump off the positive side of the coil.

I don't know whats related here and what isn't, just tried to give as much background as possible. Any ideas what could be wrong?

Thanks in advance
Darren