I have a 1997 F150 with the 5.4. I get heat fine for the most part. The problem that I am having is when I turn the fan speed down after a while when i turn it back up to the high setting almost nothing blows out. I can hear the fan running as if its back on high but I don't get the hot air movement. If I let the truck set for a couple of hours and go back out I start it everything will work fine. What should I look for.