I can drive on highway and it does fine but when I start to drive around in town it start to get up to about 220 but if it's hot out it gets about 230-240 and when it sits and idles like at a stop light it can get up to about 250, I have already replaced the thermostat, flushed it and put new recommended antifreeze in it, also my antfreeze seems to be dissapearing, but it's not leaking antifreeze anywhere that I can see, what could be the problem?