im assuming this may be normal based on my dad owning the truck for the last 5 years and he said its always done this.

when im pushing the truck a bit, to pass or hauling something etc..., my AC shuts off. it almost feels like its blowing a bit through the floor vents. then i can hear the plenum popping back open and it starts blowing through the dash vents again.

my truck is a 98 V6. is this common? or is it a plenum issue?

 

wild vents. its a the check valve in the engine being bad. under load the vents lose vacuum and it'll switch to defrost.
here is a tsb for it http://dodgeram.info/tsb/1996/24-01-96a.htm

 

hmm i need to look into this

 

Search on here like 420 said. It's called Wild Vents. I have thread on it, and my solution worked for me, but it might not work for everyone else.

 

I have the same problem, but compounded by a vacuum leak somewhere in the A/C system. When I turn the A/C off, the leak goes away.

 

Mine already has a check valve but it's hooked in the line BEFORE the reservoir, CC and HVAC. The CC has always worked fine but I still lose the dash vents under load. And the check valve is on the correct way.