I have a Sondors Thin model with a 350w 8fun motor (Bafang BBS01). The motor makes an occasional whirring sound then the power is nonexistent but the bike's LCD stays on. If I turn LCD off and back on the motor comes back to life. On a 30 mile ride this happened at 3 different occasions during the first 10 miles. No problems for remaining 20 miles. Any ideas what this might be?