sounds to me like the problems that some have been reporting on the MGMs. As soon as you give it to much throttle it goes into overcurrent safety and limits the rpm. an 8xl on 4s should be a fairly powerfull setup. More powerfull than any nitro thats for sure.
Check this thread. It may have some info you can use.
http://www.rc-monster.com/forum/showthread.php?t=10153