Mine (droid original) is constantly switching back and forth, regardless of texting or not. But, I'm I mostly notice it when I'm at my office (a steel frame building in a known bad cell area), or in my basement at home, so I kind of expect spotty coverage.
My issue is that when I AM texting in these areas, sometimes my message will get stuck sending. Then I have to wait for it to eventually time out (and give the not sent error) before I could do anymore texting. This time out seems to range from about 1 minute to 3 minutes (on my averages) - but at least twice now it never recovered and had to power-cycle the phone. I would love for there to be a way to force the phone to cancel the send before the timer - then I could resend. It is so frustrating to sit there and watch it do nothing while holding me hostage.
Oh, this was happening in the last 2 updates, and happens regardless of which messaging app I'm using.