Bizarre 2005 A/C Problem

So every once in a while, my A/C starts to sound like it's struggling. The force of the air gets stronger and weaker, and it starts to make a whirring noise (much louder than it should be). The temperature is fine, it's just the blowing air.

It will continue like this for weeks or months, then eventually when I start the car (particularly if I'm in my parking garage at work for some reason?!?! - could be a coincidence) the A/C won't blow at all. If I hold my hand near the vent, I can feel cold air - but it's just sitting there, not getting blown into the cabin.

However, once the car gets up to 60-70MPH, or if we hit a big enough bump, whatever is stuck or broken gets jogged and the A/C starts blowing again and is good for the rest of the trip.

I have taken her in seven times or so over the past three years for this problem. They'll replace a cooling pump, or remove twigs, or tighten something, or replace a seal, and then the A/C works and sounds GREAT for a week or two, and I think it's finally fixed, but then it starts sounding unhealthy again.

Has anyone encountered this problem? No one at the dealer knows what it is...

Thanks!