Occasionally when I'm driving my 2007 Silverado, the message center says, "lower speed". Then the truck automatically slows and you can't go more than 40 mph. I pull over and cut it off, wait about 2 minutes then crank it back up. It then runs fine. My mechanic's computer said it was the accelerator or the cable. Is this common? And is this accurate? Also, when it slows itself down, it sounds like it's missing. One mechanic said it was the valve lifters and another said it was the rear main bearing. Any suggestions?