Issues with Dexcom

So I got the Dexcom in early May, and for the most part, it's been going fairly well.  My biggest and most consistent complaint is that quite often, I get those little question marks, and they stay for a really long time, and don't go away until I change the sensor.  That doesn't even help sometimes.  I've had no issues with getting replacements for the sensors (I have to pay quite a bit of the charge for the sensors), but it's still an issue and I'm getting really frustrated.  

Does anyone know if this is related to the sensor or to the receiver?