As the speed issues are sporadic with me (mostly slow, sometimes fine) I did a dozen tests with pingdom. Out of the twelve tests, three of them took between 47 and 55 seconds for the domain to be reached but then the site itself loaded in quickly, at around 5 seconds for the forum home page to fully load. The rest of the times it was around 6-11 seconds for the domain to be reached. It could be a DNS issue where a drop in packets getting to the DNS server is causing a delay, but it may still be a queueing issue. With only three out of twelve tests showing the slow-down, however, I don't imagine your hosting company will worry enough to look very deeply into it.
It's definitely not a caching issue with me as I run Crap Cleaner every morning before I start work and again at around 6pm to get rid of any crud I've picked up throughout the day. OS is Windows 7 64 Ultimate, and I use a combination of Firefox (mainly), Chrome, Opera, and Safari. It's actually slowest on Safari, but that's the browser I use the least.
I'll just soldier on.