volt meter in my truck has always been in the middle at 14v, recently i start the truck runs fine put it in gear and the voltage drops to 9-10 drive a little shoots up to 12 or so drive more drops to 9-10 and bounces back and forth between the two, and also starts to mess with my other gauges as well. when i turn the lights on it gets worse as well. i got the alternator and batteries checked and they work i replaced the terminals and leads to the battery but still doing the same thing...what could it be? thank you for any help

 

what year?