Temperature rising when just ticking over for a long time not unusual, especially if the ambient temps are a high. All 1800 variants run hotter under the hood than, say, the Amazons and 140', due to lack of space. Water pump could be getting a bit tired and not as efficient as it was when working at low revs. Cast impellors can rust away a bit if run with water or old coolant. Try some "Water Wetter" in the coolant. Check that rad and bottle caps are correct and sealing properly.
As said, if you have the clutch fan, make sure it's working as it should.