Every 603 will exhibit the behaviour that you describe. Some of the better ones, with a very good cooling system (excellent radiator) will top out at 100°C. Mine has been observed to hit 105°C. on a very long (5 mile) upgrade in 80°F. ambients.

If your radiator is not all that terrific, it could climb above 105°C. under continuous load. But, not a problem. That's why you have the gauge. If the temp. reaches 105°C., just lift your right foot and climb the mountain at at 2000 rpm at 45 mph. The temperature will immediately stabilize and will probably start back down.

The fan clutch should be fully engaged at 100°C. This normally holds the temperatures below 105°C. if the clutch is functioning properly.

If the temp climbs above 105°C., the electric fan should kick in and assist.