This is a little know issue that has not been resolved as far as I know. Myself and a few others I have come across while snooping the forums have also had issues with.
We have been in touch with Joe at Castle, but that was a while ago. He has been great in forum convo's, but must be a bit tied up at present to sort the issue.
I had wired for high-power mode and got the same trouble with my Reedy 19 turn mod motor. Worked great in low-power mode with the old Tamiya 23 turn, but even that in high power mode would not work properly.
A few things I have tried that didn't work (so you won't waste your time):
1. Hard-wired the motor to the esc.
2. Made sure there were 3 capacitors on the motor.
3. All my wiring is Deans Wet noodle 12 guage, no current issues there!
4. Tried all 3 power wires soldered at the same point to the motor wire, as well as "staggering" them back from each other.
5. Hard-wired the battery to the esc.
6. Tried re-setting controller end-points, and using various end-point settings.
7. In desperation tried an AM radio set.
8. Tried a Tamiya 23 turn super stock (can't draw much less power than that puppy).
9. Twisted and wove the motor wires to eliminate EMF.

I use NiMh still, but they are good batteries and work fine with my 12 turn mod motor and the GTX esc, so I don't think that is an issue.

I don't have Castle Link, and don't think I should have to buy it to make the esc work properly.

I will post back on this if Joe gets back on the other forum.

Feel free to try anything I didn't think of and let me know!

I'm a little PO'd about it all and have ordered a Tekin R1 which is on it's way. I'm not recommending the Sidewinder until Castle can either enlighten me on how I cocked something up, or tells me how to "fix" the issue