powerbrake-stepping on the gas and brake at the same time while in D. it is also called brake torqueing. not a good idea. eventually you will change the axles and brake components
Even though I've only driven my parents GTS once, (and have been in it once), I think that you shouldnt shift between 6k and redline, because it seems to lag a bit going into the next gear. Not sure if thats most def what happening but, thats what I've experienced.