MG,
I think you play a lot more aggresively then I do, which invites more profit but a lot more varience. I'm selectively aggressive in choice spots. I play 4 tables so it's more about the hands then actually playing to win pots. I'm only about 12% VP$IP and my varience is very low because of that. I think i I have about $60-$70 standard deviation an hour on 4 tables of .50/1.00 NL. Prima's passive players also help that number. (I told you poker is a grind for me

)
The 6 buyins thing was just a bad run of horrible river beats ( the weekend.) I've only been playing total pro for a month now so I can't say how often it happens to me. Last year I played over 200 hours on Prima and I know there were 3 or 4 times where I lost 4 buyins in one or two sessions. I was playing more aggressive then with a VP$IP of around 30% though.
On Party I can lose $400 in an hour playing two tables of $100 NL. That's why I left that hellspawn of poker.

I can't handle those kinds of swings emotionally.
I think you should mix up your game a little. Switch sites maybe, just play top hands for a change, be a little less aggressive, and see if that gets you out of your slump. It can be hard to outplay some of these people on our limits. Reason being, they don't like to fold. You may represent a set on every street, but they'll call you down anyway with second pair. That's the reality of .50/1.00, and 1/2. These guys are playing for fun and want to gamble.
Hope that is some help to you. I wish I had more experience to tell you exactly what to do in this situation.
one more thing:
Do you think your live games with deep stacks, may have affected your online game in a negative way?? That thought just occurred to me.