I have a 99 Expedition EB 5.4L and my heater has started to act up. When I stop at lights it starts blowing cold, but when I rev the engine it gets hot again. Is this a blend door problem? I found a site that has instructions for this, but I don't want to get into it if this isn't the problem.

 

Sounds more like a flow problem thru the heater core, it may be plugged up.

 

Check your coolant level, sounds like it may be low, or your waterpump is not putting out properly.

 

I would suspect low coolant level as well......

 

The most common problem on these are the blend door actuator!

 

The blend door actuator would be constant problem, not changing with engine rpm.