Doing my best! I have looked at all the "HTML suggestions" and addressed each of those... Also, like I said, we are a 12 year old site with NO problems ever. Google has always seemed to like us.
Do a google search for "webmaster guidelines" and you should see their help page. Broken down into three sections G say that violating one of these can cause a problem. I know you haven't had any problems before but it's a good checklist.
Possibly, but not bad. Goog shows us at 5-8 seconds a page
Looking at "Site performance" in GWT they say that 1.5s seconds is the maximum load time for a "fast" site. So might be worth trying to get this down, but I agree - unlikely this will be causing the problem
Oh, I forgot. Over the past month, I am starting to see a dip in Yahoo... does Yahoo follow Google in any way?
Not in my experience - the results between G and other engines doesn't seem to have much of a relationship.
Do you see any geographical trend in the visitors you are losing?