Do you have a fuse on the pull switch for the headlights? If it is fused, and you loose power to the headlights due to a short (ground), the fuse should burn, and the lights will not recover. If there is some kind of thermal fuse, then recovery may be possible, but I've never seen a thermal fuse used in a vehicle circuit.
Consequently, something is intermittently interrupting the power to the headlights, and it sounds like you are getting a intermittent ground or a loose connection. Loose connections can overheat and intermittently interrupt power (and also cause fires due to high resistance at the loose connection).
The dimmer switch could easily be the culprit--again due to a loose connection (with the attendant overheating), a false ground (also a potential source of extreme heat), or a ground in the dash indicator for the high beam circuit.
The headlight switch itself can have worn contacts that fail under the additional amperage of the high beam circuit.
Go find the problem and fix the offending rascally part or connection.
