Normally if it gets hot when your cruzn it because your cooling system is undersized, hence like the radiator is to small. You have all the air you need at cruzn speed.
Getting hot at idle is a different story, could be two fold, 1st and foremost - not enough air.
Second, low coolant flow at idle rpm
Both very common problems
Go a head and try your 180 thermostat, i doubt it will cure it, but you never know until you try.
I run a Milodon High flow stat 160 in my 340 with factory A/C, even on the hottest days it rarely gets above 180-185 at idle, never gets above 160 at cruz speed.
