Performance losses at temperature

[veetwin]

Ok, so I have removed the Pipercross cone filter and replaced it with the original airbox fitted with a K&N panel filter. When the car is up to temp, at around 84°C the car starts loosing midrange power, quite noticeably. Any idea what may cause this? I used to have a Cosworth that did the same due to excessive turbo heat, but I would have thought that wouldn't be the case with the Liz.