You burp a system by leaving the cap 'off' and allowing the thermostat to cycle through, a few timesd, and then adding more fluid. bascially, jsut isnure you get all teh air out of the system.
what is your thermostat temp setting? before putting a thermostat on a vehicle, place it in pan of water, on your kitchen stove, along with a thermometer, to insure it is set properly adn functioning as designed.
10-15 degree above thermostat setting is not unusual,for engine temps, but should drop down, once system performs and exchange of fluids, form radiator to the engine.
remember, whatever temperature the thermostat is set to open, the fluid must pass from teh engine, to the radiator, then the radiator fluid, which is now cooled, must pass into teh engine, so an engine running slightly higher than the thermostat setting is to be expected. again, the engine should cool down rather quickly though.
if not, my concern would be blocked passage ways within the engine, not allowing a complete fluid exchange or restricting the fluid exchange OR blocked passage ways within the radiator itself.
Placing a thermometer in the raditor, with cap off is another good way to see jsut what teh temp is, of teh fluid coming out of the engine. and seeing what the temp is, of teh fluid right before it goes back into teh engine. either the flauid is not doing a complete fluid exschange or soemthign is preventing it from cooling properly while in the radiator.