[meaubry]

Remember to check your battery under load - a battery will test as though it is fully charged until a load is applied. Check all connections and try using a different charging unit.
It doesn't sound like a motor problem, look at the transmission and your chain routing as too much drag can cause problems too - due to the motors having to work really hard to rotate the wheels.
You mentioned that your gear ratio was nearly identical as last year - relook at last years machine and make sure you didn't miss anything (a gear in the tranny for instance), because it sounds like you may be geared wrong or there is additional friction in your system.
Put last years machine up on blocks and measure how much force it takes to rotate the wheels, then do the same test with this years drive train. I'll bet you will see a big difference.