It should increase at idle. You can try another thermostat. 180 or 185. If you have a 185 in it, while driving it shouldn't go higher than 190-195 on a real hot day. When it's cooler out and you have a 185, then the temp won't go above your 185 thermostat. If you do then you have other problems. Do you have 50/50 water/antifreeze mix? When was the last time you flushed the cooling system? Got a shroud? If you have an aftermarket water pump that's pushing more water than a stock pump then your pump is the problem.