Wow, I am testing a 2005 crossfire from a dealer. Just 51km It has the EXACT same problem described by many!!! Cruise shuts off over 60 mph. Yes it as miss matched tires. .. Will let you know if matching fixes. I have degree in electronics. The cruse, speed control, traction control could be VERY sophisticated! ... I'm impressed.

 

As the Crossfire is an SLK2320 under the sheet metal I checked what the slkworld.com forum members experienced around this problem. Forum members there found that incorrect tire sizes caused the cruise not to function correctly at higher speeds. There was confirmation that installing the correct size tires fixed the problem.

 

I've got 235/40/18 on the front and 285/35/19 on the back. It has not affected my cruise control.