I've seen this with a local airline, but in their case I believe it's due to the relative scarcity of seats on a flight. If I start the processing purchase, I

I've seen this with a local airline, but in their case I believe it's due to the relative scarcity of seats on a flight. If I start the processing purchase, I can see prices go up in another tab. If I finish the process (purchasing the tickets at the original price, mind), then the prices stay up. If I bail out and wait ~10 minutes, they go back down again (10 minutes is the length of time your flights are reserved before being released again). This fluctuation doesn't happen all the time; looking at the "book your seat" page, it seems to be correlated to the number of seats already booked and crossing some threshold. All of which may not at all be applicable in your case, but I thought it was interesting (/sensible) :).