Trigger point problems are usually intermittant. The problem can also be in the harness connection. Next time it happens identify the cylinders involved and watch that they are the same each time.
One problem that isn't exactly electrical is that due to constant pressure from one side (due to the ignbition points) the distributor shaft wears in one direction. The movement can reduce the amount certain of the trigger points get moved. A good reason to install an optical triggered ignition.
Bosch Master, ASE Master, L1
33 years MB technician