Yes, I've been using several including Pingdom to check loading times. But this has been a very screwy problem in that I'll check a page and get a loading time of 4.2 seconds, then a few minutes later the same page takes 45 seconds.
As for plugins, I just disabled everything except Akismet and that helped tremendously, so I enabled Hyper Cache, "DB Reloaded Cache Fix" and All in One SEO. As of now I'm getting a consistent load time of 4 seconds, so that may have been the problem all along and things are looking better now.
I'll see what GA comes up with tomorrow for today's Site Speed.
(The bad news is that I've already sent donations to the authors of all those nifty little plugins I was using. Oh well...)