I'm in the Philippines and I have same issue here, mostly in the town im living in. In other countries I dont have seen the problem.
So I assume its the flacky network here ...
Its goes from GMS - gprs - edge - EDGE and back to GSM
Sometimes only GPRS (probably not a 3g site).
Sometimes it just goes to off
Usually this happens when the network here is busy.
Nothing to worry about, just very annoying