I inserted a new sensor this morning. I waited the two hour start-up period, and then when it alarmed for the two bg readings, I entered them (186, 176) and the Dex put me at a 184. It was reading really accurately (as did my last sensor) for hours, up until about 6pm. That was when it gave me the "???". I was like, okay, I'll just wait it out and see if it starts reading okay again. Well, it finally started giving me readings again, greeting me with a "Under 55 Alarm." I tested and was 39. Didn't feel it. So it continued to give me readings for about a half hour longer until I got the "???" again. I have had "???" for over an hour now. So, I know why it gave me the "???" the first time; because I dropped from 244 to 39 in about 45 minutes, but I'm not sure about why I am getting it now. Is it because my bg is changing really fast because of the low again? Is the sensor bad? The last time it kept giving me this, my sensor failed the next day. Any suggestions on what to do?