This sort of problem happens to me all the time, no matter what browser, operating system or type of Internet connection I use. Usually, if I wait a few hours and try again, the problem disappears.

In those few situations where the fare is quite low and I want to book the ticket immediately, I will give Web Services a call. Even though I know it won't help, they always make me clear cache and cookies, restart the browser, and try again. Then when that isn't successful, they give it a go themselves. Only when the transaction fails on their own machines will they attempt to make the booking and force the price. It always takes a while.

If it helps, the only "trick" that sometimes helps is to make the booking without being logged in. But that only seems to work some of the time.