It’s important to realize that these are automatic price tiers in the system. Once all of the K fares are sold out, the cheapest tickets automatically revert to the next highest fare, T or V or whatever it’s called on your airline. So often when the price you’re looking for shoots up all of the sudden, it’s often because Tuan over in IT just snagged your ticket out from under you.
Orbitz just updated their software with this sort of availability checking. You’ll notice when you book a ticket that it will say “Hurry! Only x tickets left at this price!” That’s them automatically checking the fare buckets and telling you that there are only that many tickets left in that category. So although it’s in their best interest for you to book your ticket immediately, in many cases it’s also yours as well.
Fortunately, most airlines have a return policy on their tickets within the first few hours of purchase; typically this is set at 24 hours, but will vary by carrier. So if you see a ticket that you’re pretty sure that you’d like, go ahead and book it. After you go home and sleep on it, you can decide if you really want the ticket.