Same thing happens to me. I'm on EVDO all the time and it's constanltly switching from EV to 1x when sending/receiving. The battery depletes at an alarming rate. I don't know if I want to set the mode to 1x permanently because I use a lot of data during the day too and I don't want to use 1x instead of EV since it's so much slower.
Overall pretty frustrating
