1. No, I wouldn't crash while you are dry hopping - it needs to be done @ ~ 65 - 70F to extract the best hop flavor and aroma..
2. Are your OGs consistently low too, or just FG ? If you regularly are a few points low on your OG, it would carry over to making your FG low too.
3. Yeast attenuation rates are an estimate at best - a lot of factors go into determining what your actual FG will be - grist, mash temp, yeast health, pitching rate.
4. What temp are you mashing at ? Since it seems to be an AG problem and not an extract one, I wonder about the accuracy of your thermometer. If your thermometer is not very accurate, you could easily be mashing several degrees cooler than you think. This would make a more highly fermentable wort and would give you a lower FG than you anticipated.
5. What yeast did you use? Going from 1.084 to 1.016 is 80% attenuation and would not be out of line for a number of yeasts.