Has happened to me, depending on the area I am in. I believe the default setting is GSM/WCDMA networks, which allows it to switch when you are in various coverage areas.
The strange thing for me is that it sometimes happens to me and not my wife (when we are in the same place, we both have MT4G's). That's what drives me nuts. She can be humming along at 3G/4G speeds and I will drop to E or very occasionally G.
Doesn't happen often though.