Intermittent loss of brakes can be caused by the wiring on your axle!! Most common routing of the wires going from one side to the other is through the axle. Check and see if they drilled a small diameter hole in the axle to run the wires to the other side. After continuous bouncing, the insulation on the wiring will wear off even inside the axle tube where you can't see it. Eventually a bare wire will allow the brake voltage to "ground out" and no current is applied to the brakes!! When I heard of this common problem a few years ago I removed the wire inside the axle tube and tie wrapped it to the back side of the tube then reconnected the wires with solder.
Agree with grease being on the brake shoes but in my experience this just causes a constant poor brake actuation. Sudden or intermittent loss of brakes is a poor connection or a chafed wire somewhere!!
