Was the "race" from a dig? If not there is a flat spot in the powerband on these cars (MBs too) where it falls on its face. On paper it should be a really close race.

http://benzs.blogspot.com/2007/12/ja...tail_4912.html

Last paragraph states 0-60 in 5.1. Also your intercooler water pump might be on its way out. As the air intake temps rise timing is being pulled so you won't be making the max amount of HP.