If you haven't already one thing to check first on sudden power
loss is the sliding bar in front of the power wheel. There is a
screw down the center of it that can back off and render the power
wheel pretty much useless. Mine and my son's both have had it happen.
Simple fix by tightening the screw to where the slack disappears on
the sliding black bar. You just put your finger on it and make sure it
doesn't slide or have play in it. A dab of loc-tite helps too.