Its fustrating I know, when i'm handed a work order for a vehicle w/ similar symptoms I cringe... the most common of the blend doors to malfunction is the temp selection, the higher RPMs pull more vacuum, maybe your losing it just enough to give poor heat. Did you check the vacuum reservoir, it should be located right corner somewhere below the headlight? It could be a circulation problem but its hard to diagnose via Forum. Do you have a vacuum diagram? Google " images" and work from that, your model has 5 or 6 photos of the vacuum routing on there. You have 2 different size heater hoses, i'm sure you checked that, you burped the coolant sys right? Did the PO Maintenance the coolant sys correct service intervals . Let us know how your doing.