I ran a brief test of the theory that the more interested you are in a certain route / flight the higher the price goes. So I chose the airline most likely, in my opinion, to indulge in this sort of practice, and a route which we've booked from this IP address in the past.
Right at the beginning of the test I had a computer restart coincident with the price dropping from 260GBP to 144GBP overnight. Suspicious, I thought. However I kept plugging away at the booking engine every day for a few days.
Then I used a different browser via a different IP address - and got the exact same prices.
The massive overnight drop was just coincidence, I think.