I'm having problem with the tachometer on my truck. just the other day at idel it was reading about 500 to 700 rpms( which is normal). well here lately it started wanting to run about 200 rpms idel and and at normal 60mph crusine speed its about a grain and sometimes lower. anyone have any ideas what could be the problem? by thy the wy the truck is running fine to so i don't know if its a sensor or guage problem.