All,

I have a 1999 Explorer 4.0 OHV.  Mileage is around 135,000.  Had the heads
replaced around 120,000.  Both heads had a fine crack.  This was done about
14 months ago.
Fast forward to 3-20-08.  While driving the Explorer after it had been
sitting for a few hours and engine was cold, the temp gauge would rise to H
and go back to the mid point where it always would rest at.  When I reached
a safe spot to pull over, I checked under the hood and my radiator cap had
released some coolant.  After it cooled off, I checked my level in the
radiator and the coolant tank.  Both seemed fine.  Started back up drove for
until it warmed up, same scenario, except this time, it came out the coolant
tank.  Now, it has not overflowed out of the radiator cap or coolant tank
anymore.
What I have done is replaced the radiator cap.  Replaced coolant lost during
the times it overflowed.
Will replace the thermostat today thinking that it may not be opening up
like it should.
Any ideas on what else would cause it to heat up rapidly and steady off on
the gauge.  I thought the gauge may not be reading correctly, but if that
was the case, would that cause the engine to send coolant out the tank or
the radiator cap?
I was thinking the heads that were replaced may be having issues that are
the root cause.  If the heads were cracked, again, or gasket issue, would
that let combustion gas into the coolant and cause it to heat up? But, if
that happens, why would the temperature gauge go back to reading normal?

Thanks for any suggestions.

Lance