Ok, I've been having the same problem for a couple of months with my Bionic, and didn't have the problem prior to that. Contacted tech support, tried several things, and even went as far as to have them send me a new phone...only to have the same problem with the new one. Finally called tech support again a couple of weeks ago and had a less than satisfying conversation with that guy. He suggested that I live in a limited service area (which I don't) and finally after agreeing to disagree, his only suggestion was to possibly try changing out the sim card. He didn't think it would help but had nothing else.
Anyway, when I went into a local Verizon store in Portland, OR and explained the problem, the guy knew exactly what I was talking about. He said that all the 4g phones were doing it, and that after some extensive research on the Droid forums, he had discovered that this is due to a software problem in the towers which causes the 4G connection to prematurely time out...thus causing the drop in service. He told me that they were supposed to do a software patch update for the towers to solve that problem on or around July 27th. Not a phone software patch, but one internally for the towers.
I had them exchange my sim card anyway, but was hopeful that I would soon see an end to this issue, but here we are two weeks later and I'm still noticing the problem. Hopefully what he said was true and that there's a light at the end of the tunnel. Not sure if all this is true and accurate, but it's what I was told.
