random power

I have a BBS02B (36V, 350 Watt) kit with an 850c display fitted to my Cannondale Trail 2 29er. I have it set up for three assistance levels (50% level 1, 75% level 2 and 100% level 3). When riding, the power meter rapidly changes between any value between 0 and around 600 Watts. Normally, level...