It sounds like somthing you would hear somone say if they had stock springs and a aftermarket cam..... My buddy had this hapen to him in his MR2. His A/F was way off and he had an Exhaust leak, the leak caused air to get in and mix with the unburnd fuel and caused it to ignite in the exhaust chamber while the valves were open and it messed alot up..... What type of plugs are you running, sounds like you need to read the tread about the Hot and Cold plugs for Turbo.... Colder plugs could solve part of the problem...