the power loss problem is often down to sticky veins in the turbo, its worse in colder weather because the air is denser & the turbo can create more boost, the problem normally happens in higher gears when the turbo has more time to generate more boost. sometimes if its in the early stages then a fuel treatment can help, if not the turbo may have to be removed and overhauled, a company called 'Midland Turbo' can do the VNT turbos.