Thermostat is highly unlikely. It's function is to close and curtail
circulation within the engine until operating temperature is reached
then slowly open to permit circulation to the cooling/cabin heating
system.  If it fails closed, the engine overheats and will be
apparent on your temp gauge which has it's sender in the thermostat
housing. If it fails open it slows the engine coolants rise to
operatingt temp.

Your situation is one of circulation (as in little circulation at low
rpm) not temperature as regulated by the thermostat.

On 6-Jan-10, at 11:42 AM, Automatic digest processor wrote:

> Date:    Wed, 6 Jan 2010 13:19:35 -0500
> From:    David Prendergast <dprendergast@TRIAD.RR.COM>
> Subject: Re: No heat at stoplights.
>
> The coolant bottle is full with no leaks anywhere, someone earlier
> said it may be the thermostat and the symptoms do match (thermo is
> about 4 years old).  It blows hot out of both rear and front heaters
> until I stop at a light then it blows cold.  Not really a big deal
> since the weather is relatively mild here in central North Carolina.