Ford Repair: hvac, vacuum problem, ac system

Question
I've been having some trouble with my 1999 F150's heating and ac system. it's a weird problem but for some reason whenever i'm driving around town or not on the highway my heater and ac depending on which one i have on don't always work. for instance say i want to run the heater after i have let my truck warm up and i turn the heater on it blows cold air well after i've been driving for little bit the heat will kick on not always but sometimes now if i'm driving down the freeway and i want the heater on i can never get the heater to work it's almost like i have a vacuum problem somewhere that's causing it to not switch from the ac to the heater or from the heater to the ac i'm a mechanic i work on construction equipment so i'm not very familiar with the hvac systems on cars and was wondering if you have ever seen one do this before i replaced the control panel for the hvac system thinking that maybe one of the switches was shorting out but it didn't fix the problem if you have any suggestions i would greatly appreciate

Dan

Answer
first i have to ask if you made sure the antifreeze was full ? when you turn the heat or a/c on does it blow out the defrost position only ? if both above are o.k. then yes, your leaning to wards a vacuum problem. there is one main vacuum feed fromt the engine that goes through the fire wall and then into the car, usually white or black in color. it is very easy to follow this line into the vehicle and look for kinks or breaks in the line once it enters the vehicle. give it a quick visual and get back to me