"You said that it sometimes goes days or weeks and works just fine, then for no apparent reason it stops blowing cold. You also said that it always works when driving and only stops working after the car has been sitting. I assume that the car sits sometimes during the days/weeks that it keeps working, yes?"

Yes!

And that's the strange part. It will work for a week and then the cold air stops coming, I'll re-charge it, sometimes it works for a day, sometimes a week...there is no pattern here.

I do not have gauges, except for the one in the recharge kit I purchased from Wal-Mart.