Have you removed everything that is plugged in to any outlets fed from the GFI? If you have then you may have to pull each one and look for any loose/frayed wires in each outlet until you find the one that is causing the problem. Remember what a GFI circuit breaker does. It trips at the slightest leak of power to ground so you are not looking for a dead short but more likely a slightly frayed wire that is just barely touching from time to time.
Make sure that you find ALL outlets that lose power when the GFI trips.
Also look for any outlets that may have overheated at one time from a portable electric heater.