Tires: Tire leakage, honda minivan, road surface

Question
What would cause a tire to leak ONLY when driven?
(tire has been on the vehicle,a honda minivan, for a while with no trouble, until recently, it leaks very slowly,until driven,then i'm down to 10lbs or less in 20-30 miles)

Answer
Shane,

"What would cause a tire to leak ONLY when driven?"

The same things that cause leaks when the tire is static.  Driving merely makes the tire flex.  It bulges out, it moves relative to the road surface, it squirms on the rim.

But most likely your problem is the last one.  You need to spend some time immersing the tire to find the leak.  A child's pool  - and some bricks - work very well for this purpose.