HAd the same problem. Took a while to figure out, but it turned out the fat battery-cable from the battery to the starter-relay was broken (on my bike that is the only cable from the battery plus. At the relay it splits between the relay and the rest of the circuit). As ussual with electrical problems sometimes the problem was there, and sometime (ussualy when you were trying to find the fault,) it wasn't.
The headlight dims when you rev up because the voltageregulator kind of works in steps. When you idle it doen't do anything yet, but when you rev up and the voltage goes up (and the battery is not there to 'help' to regulate) the regulator kicks in and the headlight dims.
Peter
(I have a '88 XT600Z Tenere)
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