[Healeys] HS4 idle issue

[john spaur]

When I first start the car and while it warms up the idle will be 
fairly steady. After the engine heats up to full operating 
temperature, while I am sitting at a stoplight or just stopped, the 
idle will drop down until the car stalls. It seems to happen after 
the thermostat opens up but I am sure that is unrelated to the problem.

What could cause the idle to change like that?

John Spaur
'62 BT7