I have 2008 5.3 Silverado, love it, never a problem, but my heat suddenly started acting strange, if I have truck stopped the air blows cold, once I am moving it heats up. I can leave it running for an hour or if its running all day and I am stopped, same thing. Been doing for months but now its cold and I need heat. The temp gauge is where it should be so I thought not a thermostat issue, its full of antifreeze so that cant be issue. Blend door maybe?? Anyone had this issue?