If there is no load on the gfi's and they're tripping on their own it's going to be one of three things..
1) Condensation building up within the gfi's themselves.
2) Line voltage disturbance. (What manufacturer made the GFI's? Some GFI's will trip when power is removed or "browned out", so that you must reset them after power has been restored.)
3) You got a run of defective gfi's.
If the gfi's will allow you to reset them and function, even for a few minutes,I don't see it as a line/load mix up. They shouldn't reset at all..
BTW make sure if there are lawn sprinklers in the area that they aren't hitting your pedestals.
P.S. If all else fails, try a different make of GFI, and stay away from any that say "3Grace" or "General Prohtect" anyplace on them. These have false UL labels on them (There's a thread about when I came across these before someplace in the forums) Stick with the known manufacturers (P&S, Bryant, Leviton, Hubbell, etc..)
