Temperature around 87-90 at speeds above 20 mph and temperature as high as 105 C when stopped with AC on, indicates that the Aux Fans are not working properly.
The Aux fans maybe be turning but at a lower speed than required. Possible defective fans and/or pre-resistor.
JimF has instructions on how to start the aux fans at a lower temperature. He also sells a "cool harness.
http://pages.prodigy.net/jforgione/MB_S500.html