So, I noticed a little problem with "timmy" today. I went out to start it 10 minutes before I was leaving for work, turned the heater knobs all the way to hi, expecting to get in a warm vehicle 10 minutes later... Well, I got in and it was as cold as it was outside (9 Degrees) Air was blowing ice cold, and I assumed it was just because I was used to how fast my commuter car (nissan 240sx) heats up. So I lowered the knobs, and started driving. Almost instantly, I felt the heat kick in. so I blasted it again. Great! So I pull up to the coffee shop, and while I'm idling in the drive thru, I notice my heat turn slowly to cold... Start driving again, and it heated up again! What could be causing this? I don't know much about heater cores, but could this be the issue? Any ideas, or suggestions about how to diagnose this would be greatly appreciated... I'd rather not have to put my truck in neutral and rev it at red lights to keep my heat on.