I've changed the thermo and flushed the radiator. My fluids are where they should be and the hoses are all new. The temp is fine for the first 20 or 30 minutes each day, but then starts climbing up and into the red. The heater blows nice and hot when the temp is normal, but blows cold when the temp starts inching upwards. Could I have a tempermental water pump? Would a clogged heater core cause this? What else affects the temperature like this? I really don't want to take it to the shop, where they usually ask for my first-born before opening the hood. Any advice is appreciated. Thanks!