I've notice that sometimes when I pull up to a stoplight or into my driveway, when I stop, the fan kicks on, but surges rhythmically--spinning up and slowing back down repeatedly about once every second.

When I first got the bike the first thing I checked was that the fan blades spun easily (they did). This problem is intermittent--sometimes the fan spools up and just stays on like it should.

I thought it may be related to power draw (say, maybe the turn signals are interfering with it) but on further investigation it can happen whether my turn signals are on or not--and it's not in rhythm with the signals, anyway.

Possibly a thermostat issue? Is there a hysteresis adjustment for the fan?

The fan should normally run at a constant speed.
I`ll guess you start to get a bad connection in the circuit.
A common problem is melted brush holders.

Inge K. wrote:The fan should normally run at a constant speed.
I`ll guess you start to get a bad connection in the circuit.
A common problem is melted brush holders.

I've never had one apart, but this makes sense to me.