Seems like a common thing these days. I started with replacing my oil cooler because it was leaking and could smell the oil burning off. Shortly after I did the water pump because it was coming apart. While I was there I replaced those two plastic idler pullies with metal ones and changed the thermostat. Precautionary since I was in that area and hadn't done a thermostat. Flushed the radiator, heater core and burped. Still when at idle temp is normal. While traveling the temp rises. Never gets above 3/4 but cools when I comes to a stop. Ugh.