use an ohmmeter to measure resistance to ground. If there is a fault (e.g., corrosion from water leakage through a crack in the element tube), you could have some current flow to ground in addition to the current flow through the element. If it is some sort of dynamic phenomenon where the heating expands the crack, giving the one-hour delay to tripping, you could put an AC ammeter on each leg and see if a differential current appears either immediately or over that hour.
Elements are cheap and easy to replace. Just get the stamped steel element wrench, unscrew the old one, and screw in the new one. Drain first if you can, but I quickly swapped one on a full heater with a clogged drain and had to mop up only about a half gallon of water.
I also wouldn't rule out the possibility of a bad breaker or thermostat without evidence.