What you describe about the key position and sometimes will begin running after you let go the key sort of points at either a very weak battery, or someone has put a resistor type ignition coil in place where a ballast resistor coil primary circuit exists.
I have seen the symptoms you describe on some old Nissan H-20 (stage 1) engines in Komatsu chassis.
Sometimes upon failure of an ignition coil people will replace it with whatever looks good or whatever is in stock, and that might not be what is correct for the particular truck.