Sounds like the famous "MGM full brake at neutral occasionally" syndrome. A few of us have just sent our speed controls back to MGM to have the latest software put on them (v3.20). Hopfully they have resolved this issue along with a couple of other small problems.
Here are a couple of threads to have a read through that might help.
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http://www.rc-monster.com/forum/show...ght=mgm+brakes
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http://www.rc-monster.com/forum/show...9&postcount=50

Cheers Andrew