You sound just like our situation Carfac. Our 12 year old site is going through the exact same stuff and we have done the exact same things as you.
It all really started during April/May. I have spent hundreds of hours trying to figure out whats going on and have come to the conclusion that I have lost any idea as to what Google is looking for now.
First I thought they had given social networking a stronger play but after doing research and seeing that our social networking has been stronger then ever (30,000 new SU followers just in the last 30 days)
I have thrown that out the door.
Then I thought it was due to backlink profile/authority stuff, but considering in the last 90 days we just landed several hundred new links from some of the worlds biggest companies and some of them are actually referencing our site in their marketing/branding and advertising I threw that out.
Then I tried to find patterns in data but our top 1000 serps are literally changing daily. As if they have 4-5 different search engines running under the hood.
All the while im watching mashup junk take over everyplace.
Im so angry right now I could kick a kitten through a...