iZip problems

#1
I have a 2010 iZip Via Rapido with the 250 W 24V motor (mfg by Suzhou Bafang Motor Science Co.). I had been able to use the battery down to its last gasp, and only recently found a source (Ebikemarketplace) to rebuild my battery, since Currie Technology does not sell them anymore.

The problem is when I use the throttle, the electric motor makes a groaning sound, then goes silent and the motor is unresponsive and does not power the bicycle. Prior to my getting a new battery, I had noticed that a couple of times I heard the groaning noise and it seemed like there was some resistance when pedaling, but it went away quickly and did not come back, and I was able to use the power normally.

Any ideas as to what is causing the problem and how I might be able to resolve it?