Hi,
Did you check to see if you have fluid leaks at the bottom after you came back from your 100mph drive ??? I overfilled my transmission in my 400e two years back and I had a similar porblem. The fluid would leak only at high speeds from the pan and when it hit the exhaust pipes I got the burnt smell. I pulled over and looked underneath and only then I realised what was going on. I siphoned off some fluid and then no more smell.

smk_texas
98 S420 51K