I have a 92' 5.0 convertable up in alaska. still driving it maybe for a month. Before it's just to cold.
But my ? is as I accelerate my heater switches to defrost and back to the setting I have it on. is this a normal thing or is something going on?
I have checked all my hoses and feed lines haven't notice any cracks or unplugged lines.

 

I'd say there's a leak in the main vacuum line to the heater controls. They're built in a fashion in which if the system fails, it defaults to defrost so you can still see what's in front of you. A small leak in that hose could cause what you describe.

 

Ill have to check that out. thanks man