well, how to fix would be contingent on what the problem is...and that's not a very good description of a problem.
Stock v.8 engine and turbo? Sounds like your turbo is fluttering under full 4th gear load (that is if the problem is occurring under WOT). Has the driveability of the car changed at all? Still feeling full power? Also, where are you located? Sounds like it is just wastegate flutter to me which isn't always a problem.