Replied to your mechanic finding topic, just ignore that one.
I agree an ignition fault sounds most likely. They can be a bugger to track down, but should be very cheap to fix.
Start with a good look over...
Clean the terminals on the coil to be sure they are tight & not shorting.
Inspect the wiring for fraying, kinking, broken insulation etc. There only needs to be a +ve supply to the coil and a wire going from -ve to the distributor. If the dizzy has more wiring it may indicate it's had electronic ignition fitted.
If you do have a rev counter it'll be an extra wire on the coil -ve, try removing it when the fault occurs and see if it makes a difference.
You could also rig up a direct 12v supply from the solenoid terminal where the brown wires are attached to eliminate possible faults in the ignition switch & wiring: BUT don't leave it connected when you're not trying to run the engine!
Clean & mark all the plug leads (so you know which is for which cylinder), unclip the metal spring clips to release the distributor cap. Clean the cap thoroughly and pay particular attention to the carbon brush under the middle terminal. It should be spring loaded if you push. The brush runs on top of the rotor arm, so look for signs that they've been in good contact & not arcing - a regular dark circle "pencilled in" on the rotor and no pitting. You could give the brush a gentle pull to stretch its spring ever so slightly.
You'll need some very basic tools to go much further, there are some good videos on Youtube that'll explain how to set & change ignition points - I'd recommend University Motors
...do check the carb damper (black screw cap in the middle of its dashpot) isn't loose though. That can cause similar issues and is very quick to eliminate.