Cooling system questions

2002 LS V8 107K miles.

Occasionally the car overheats. Turning on the heat cools down to normal operating temps quickly (which indicates bad thermostat). After a stop (shut the car off for a few minutes) and start its cooling fine with the heat off. I know this sounds like a sticky thermostat but I have replaced it twice in the last two years. Why am I going through thermostats? Is this simply tough thermostat luck? Can the thermostats be the wrong temp range? I've gotten one from Ford and one from Lincolon so I am confident that they are giving me the correct part. Parts stores never have it on the shelf.

Second symptom. When using the heater lately, the car blows hot air on high RPM (crusing RPM). When I get to a stop and the car goes to curb idle the air turns cold.

...someone is saying "then don't use the heater"...

The car has the correct coolant level i.e. the reservoir is full. The coolant is clean and bright green like it should be.

Thanks!

I wonder if maybe you have air trapped in the cooling system from the thermostat replacements. There have been some posts fairly recently regarding the procedure for bleeding air from the cooling system.

-Rod