my clio williams as a bb tuning turbo conversion. about 2 mounths ago my injectors went, then it blew the head gasket then the turbo went and shell bearings and oil pump now with all that fixed it seems really hard on fuel, and when you hold some revs on it like 2500 it will go down to 2000 for a second and back up, it does this right throughout the rev range.any one know what could be wrong?