1999 gets hot while idle, OK underway

I have a 1999. I have changed the intake manifold gasket about a year ago. The water temp has been normal until just recently. The water temp is normal as long as the car is underway. It is when I stop and idle for a period of time, I can see the temp needle moving to the hot side of the gauge. As soon as I get underway the temp needle moves back to the normal range. Stopping at a stop light is not a long enough time for this to happen. Its only during long idle times, 15min +. I have check the fans and they work as advertised. Thermastat problem? Any ideas? Thanks

in any situation like this the first move would be to check your overflow jug.
If it is low, when the car is cool remove the rad. cap, fill to top, fill overflow jug to the cold line. remember your checking the rad. with the engine cold. The least expencive move would be to change the thermostat.
you could have one going bad and getting intermitent problems like: working just fine one minute and overheating the next, staying cool, (your situation) or just plain boiling over.
Be sure you install the new thermo. in the right position. I have seen them installed upside down and cause problems something simular to yours.

and put one that is made in the usa as the ones made in mexico are a couple of bucks cheaper but willnot open in boiling water been there done that . it is normal for the temp hand to get a little over 1/2 way mark before the fans kick on . also there are two bleeder screws that are to bleed air out of the cooling system. one is right above the waterpump on the heater pipe the other is on the thermastat housing turn the little part left until coolant comes out with the motor running.have you ever replaced the waterpump?

check also your waterpump, low idle makes the pump pump slower and faster as the idle goes higher.
idle the vehicle till it starts heating up then press the gas pedal and holdit for a while ( in park ) , if the temp starts to back down your pump is more likely the curlprit.