I have to drive almost all the way to work (20 minutes) before my heat starts to work steady. While the heat is not working, if I get the rpm's up, (say...5 or 6000) I get some heat, but it doesn't last. I'm thinking a vacuum actuated door to the heater core is sticking. Anyone had this problem? Does everyone else get full heat as soon as the temp needle goes to the middle? Thanx.