What type of speed controller do you have installed?
If it a ABB a fl2 is a dc bus overload. That means either the drive is absorbing to much regeneration energy from the bridge when it slows down, or the incoming line voltage is to high.
What is the incoming line voltage to the machine, each leg to ground? L1.L2,L3 to ground.
Also what is the voltage between each of the 3 phase legs?
If these are normal, you may fix this by increasing the decel time on the drive, call me for details on that.
Or.... your gear box may need to be replaced as that is what we realy on to slow the machine.
Hope this helps. Call with any questions
Regards
Mike