This is not uncommon at altitude. Most caps are set to hold to about 14-15 psi at sea level. I can't give you the exact altitude/pressure correlation, but, the relative pressure inside the cooling system increases as altitude increases, even w/o overheating. Solution-> high pressure cap for life above the clouds. Or, your cap is not holding it's rated pressure and needs to be replaced.
2000 Dodge Grand Caravan SE
[This message has been edited by jeffsr (edited 09-05-2000).]