Well it finally turned out there was no problem with my website or the server. It is the London test machine that has caused the problem. Normally I have always used London IE8 DSL for all my testing because it was the closest to my webserver and therefore should provide the most consistent results. However having investigated this problem further I found that in 20 runs the London test machine threw up painfully slow results for those three file on 6 occasions. For comparison I ran the same tests on Dublin, Madrid and Denver and got 100% good results.
In future I will probably run my tests with Madrid because that seemed to provide the most consistent results. I’ll also have a more open mind when I encounter “problems”.