[suicideneil]

Thats highly unusual...

Difficult to say what could be causing besides a faulty BEC like you said, though there are a few things you could play with maybe. Check your epa on the steering servos to make sure they arent maxxed out and binding- that would draw lots of current and drop the bec output voltage enough for it to spaz out sometimes (or just run red hot and thermal even). Also, try plugging the esc into the servo slot on the rx and check its opperation like that- then stick the servo into the throttle channel to see how it behaves.

Since you tried other rxs and servos it does seem likely though that its the esc to blame, which is a shame as BEC problems are pretty much a thing of the past with the MMM..

One last thought- do you have anything like a glitch buster capacitor you can plug into a spare slot on the rx? You can use a ~25-30v, ~1000-4000uf, 105*C rated item with a spare servo pigtail soldered on if you fancy making your own- radio shack will have the caps, or a decent hobby store/ electrical repair place.