Those 730 AAA batteries are not up to the task of supplying the current your SS motor demands - it wants 8-10A and those are good for 5-6A max. What is happening is that, under load, your voltage is dropping below the level of the LVC (?5v) in the ESC - thus shutting off the power to the motor. When you are at only 3/4 throttle (?4-5A) the batteries CAN cope without dropping voltage - same with the NiCds. What you need to get are some 7-cell KAN 650 packs or the CBP equivalent - they will give you 8-10A without trouble WITHOUT dropping voltage under load to the same extent.

Cheers, Phil