Tom,

Hard to say what causes the idle problem, perhaps have a look at the carb, or iginition timing. Both can cause (slight) overheating.

A high engine temp when idling for a while is quite normal. When idling there's no 'wind' blowing through the radiator, and teh waterpump is also turning very slow aswell as the fan. When reving it a bit, the fan blows fresh air through the radiator helping it to cool, and at the same time the waterpump runs faster.
In very hot weather and in town driving (stoplights etc) I somtimes let the engine rev a bit faster then idle so the temp won't go of the scale.

The 30 sec inteval of overheating/normal temp while cruising is not normal. First thing to look at is the termostat or (as you sugest) a blokage somewhere. Is there no air in the cooling system?

Ben