my 99 jeep grand cherokee keeps overheating. thermostat? head gasket?,?

[Kenneth]

My jeep keeps overheating I have gotten a new radiator recently so it can't be that and they also didn't even change the thermostat so could it be that? Sooo anyway it all happend like a week ago and I noticed the thermotat contantly going up when it stays in one place as I was to sit at like a red light or something and it overheats but then when I accelerate it goes back down to normal. Now yesterday it all changed cause it didn't go back down to normal and just overheats now. All the coolant in the reserve is gone but it doesn't show white smoke out the exaust. So no gasket? So yeah. What do you think is the problem? Also when I take off the radiator cap I see water in it and it doesn't bubble like it would if I needed a gasket but I don't know. Please help