I drive a 2015 Silverado. LML. (It has 170,000 miles). Truck is fully stock. I'm having a problem with the idle. It happens a lot but is not continuous. When parked the idle will increase to about 800 rpm. It usually sits at 625 rpms. When I put the truck through all the gears, or when I rev the engine, it will not help. It done this all this morning, but I'm in the truck now parked, and it's idling normally now. What all could cause this?