The battery is not being charged while the engine is running. It will run good after the battery has just been on a trickle charge overnight, and after a few hours it won't start back up, and the battery is dead.

 

First check fuses and connections, find regulator and check for voltage coming to regulator ( usually a red wire ). If voltage to reg is ok then find the wires that run from reg to stator ( yellow I think ). Check A/C voltage across wires, should have min of 28 to 32 A/C volts with engine at WOT. All this needs to be done with battery charged. If A/C volts are good and battery and connections are good then you most likely have a bad regulator.
Hope this helps, TORK.