The vents are run by vacuum from the engine. When under a load, the vacuum in the engine's intake drops. At that point is when you loose your vents. That tells me there is a vacuum leak either from the engine to the inside of the car or under the dash. With no leak, the system can handle a drop in vacuum from the engine. With an additional loss (leak) the vents can't stay in the position you want.
In this case, you are looking for a vacuum leak either under the hood or the dash. Many times with the engine running, you will hear a slight hissing noise that will help locate the leak.
Let me know what you find.
Tuesday, August 18th, 2009 AT 11:07 PM