I have an outdoor waterfall with a small pump that recirculates the water. It is plugged into a gfic plug that is outside, but covered and protected from the weather. I have to continually re-set it. Will cold weather (the temp is in the low 30's at night) make these things kick off? When I re-set it is usually runs several hours before the power loss. The pump seems to be acting normally.
Most likely a problem with the pump or power cord. Take the pump inside and plug into a gfci operate it in a bucket of water with same amount of cord submerged as outside at the fountain. See if it trips. If not you have a moisture issue with the gfci itself or a gfci that needs replacement.
Is there anything connected to the load terminals of the gfci?