I guess it could be a cylinder bypassing.. do you have the 3rd cylinder?  Mine has 3 and it's hard to see the leak but the inside cylinder on the double cylinder side leaks slightly and I have to bump the head just a touch every 500 feet or so.  It leaks slow enough that it gets to pushing up on the floor a little more than I'd like but not enough to trigger the AHHC to pick it up.  Sometimes I just click the head down and the AHHC puts it back normal.