[embee]

Sounds like thermostat cycling, though it usually does it rather more frequently like 30sec or something.

Basically the 'stat is at the outlet from the engine, before the rad, so when it heats up the 'stat opens, pushes slightly cooler coolant in the rad through the engine which lowers the temp at the sensor and tends to cause the 'stat to close down a little again, which restricts the flow to the rad and the engine heats up a bit, and so on.

You might want to double check that the rad is actually full of coolant, not just the make-up bottle.

Engines are developed so that the cycling is minimised usually, but bike systems tend to be somewhat crude in many respects and under certain circumstances considerable temperature cycling is not surprising.

A lot of modern cars use thermostats at the inlet to the coolant pump rather than outlet from the engine, and they blend rad return coolant with the re-circulated engine coolant, which can make the system inherently more stable (you don't need to wait for the whole rad contents to circulate before noticing a change in temp). Also "smart" 'stats are more common, which use electronickery rather than simple wax capsules, so can be controlled.