yeah, the difference in mAh between 8.2v and 8.4v on that 800mAh pack is probably only like 50mAh or less, so yes it should go down that fast.

How old are your motors? Like how much flight time? Are they getting hot? If they've got over 10 hours, or are getting hot, then replace em, and make sure to break them in first.

an hour and 45 min charge could be a bit excessive if you've got the 650mAh charger. With that charger it should take you right around an hour if you haven't over discharged the pack. (80% of 800 is 640mAh), but still that shouldn't be causing this problem.

I would say check out the motors and see if replcing those doesn't fix the problem, also if you have not seen it, check out this page with cx2 articles
http://www.heli-max.net/mediawiki/index.php/Index:HOWTO