Just started noticing this problem. Went to Chicago and noticed that the K band speed sign at the Indiana West toll road exit didn't set off the detector. However I did get Ka's and some POP and Laser falses. On the way back home the same K band signs did set off the X50. Started watching this real close and have noticed it doing it off and on. When it stops working it just affects the X and K bands and not the others. Unfortunately it failed at a small town $$$ speed trap and the LEO got me for 14 over.
Going back and looking at it the next day, he must have gotten me about 100 yds from the 50 mph sign. Haven't had a ticket in over 19 years but he didn't even flinch and wrote me one. I'd love to fight it but just one day off of work would cost more than the $120 ticket.
Guess I'll go back to my Whistlers.