Go into the settings on your Dexcom receiver, then choose device info. The third item down is the transmitter battery status. I'm also in my 8th month with this transmitter and my receiver is saying that the battery is "ok". That's the first thing I would check.
The "???"s sound more of a sensor issue, though, as @YogaO has mentioned. I usually have one really bad day during my second week of sensor wear, usually around day 10 or 11 of wearing it. If I can ride it out, it seems to straighten itself out. I just have to go without readings for several hours.