[Shotimes] Radiator Overflows AFTER engine is turned off ?

[Phil Piotrowski]

I replaced the radiator (is was leaking in the cold)
After the weather warmed up, and shortly after an oil change,
I noticed that while is stop and go traffic that the temp gauge
Was near the N in Normal (pretty hot from my experience) !
I pulled over, opened the hood, and turned the engine off.
Within a minute, the antifreeze was bubbling out of the overflow tank !
When it cooled, it sucked all the coolant out of the overflow tank.
When I got home, I ran the engine with the hood up, and the temp gauge would
get to about the O, before the fan would start and get the needle down to
about the M. 

If I am moving on the highway, the temp  gauge is normally on the L in
Normal.

Several times, I have gotten home, put the car in the garage, and then in
the morning, the 'low coolant' light will be on.
When I go to fill it, it looks like the coolant boiled out again !  It is
all over the floor.
I have checked the overflow tank, it does not leak.
I did replace the thermostat (it does have the 'giggle' value, and is rated
for 190 degrees (AutoZone said that was stock))

Any ideas as to why this is happening and how to fix/troubleshoot it ?

Thanks to all,

Phil Piotrowski
'95 MTX - "DR VMS" - 170K