Was that frame built to have a suspension fork? Or is it's geometery intended for a rigid fork that is much shorter?
When I had a similar problem, the cause was obvious.
But not as obvious as you might think. I had slop in the braces that support the rear rack. When I tightened that up, I could resume riding this bike no hands. Still has horrible balance, but the shimmy went away.
So look for anything moving around. How tight is everything inside that battery box?
THE LIPO RULES. NEVER ABOVE 4.3V NEVER BELOW 2.7V DON'T PUNCTURE
Ideal charging /discharging range for Lipo, 3.65v minimum 4.1v maximum
See battery technology section, FAQ thread at the top of the page for lipo noob info.