Need help.... The heater on my esv don't want to get hot when sitting and running. If you push the gas a little and bring the rpm up it will start to feel hot. If driving it does fine but only does this when setting. This becomes a issue when you want to warn the vehicle before leaving on cold mornings. I had to water pump and belt replaced about 3 months ago and I have noticed that the temp Guage wants to jump from half position to the quarter position at any giving time. Could the cooling system need flushed???