Damien,
Have you had your running style looked at?
The pegasus are neutral trainers, for people whose feet dont roll inwards while they are running. If your feet do roll inwards, the Pegasus won't provide your feet with support against that, and this might be causing your pain.
Alternatively, when you say "old reliables" - how old are we talking about here!? might be worth checking out the mileage the shoes have done, and if its coming up to about 400/500 miles, they might be wearing out (especially if you land heavily?)