2008 335i changed valve cover gasket and plugs with battery disconnected for several days, after repairs made car ran poor, shifted hard, steering hard, idle erratic tripping reduced power and SES light ... after 2 weeks driving steering and shifting better but reduced power and SES still come on so no turbo power, car was fine before disconnecting battery. both DIY's for valve cover gasket replace indicated the battery disconnect with no warning about computer control module losing settings, miss the performance, no fun without turbo power, please help