Werm,
Thermal cut-back (not cut-off) limits and slowly reduces the peak current delivered to the motor from your motor controller. You would perceive this on the bike as a drop in acceleration rate and eventually a drop in top speed. This protection is based on a measurement of current and temperature over a given time period with the effect of protecting your motor from permanent rotor damage. Ambient temperature can affect when this protection kicks in, but it is much more likely to be the result of repeated heavy acceleration or sustained high speeds, both of which require alot of current to be delivered to the motor. I would recommend getting in touch with your service rep as it is pretty painless to have him/her take a look at the logged data from your usb drive and take a look at what's going on when thermal cut-back kicks in...
Hope that helps!
