It's the firmware- this problem affects many people and is covered alot- can we get a sticky please ( "Having throttle issues with your Castle Esc? Read this first.." )?
You need to check which version is loaded onto the esc ( under the software tab in the castlelink program ), then try upgrading or downgrading it. Anything newer than v1.24 can cause cogging with many MMM escs, so downgrade to that version and try each new one to see if it is also stable or not.