LIttle Electrical problem help???

[71Longbed]

Hello all i currently am trying to get my 71 to run right and be my daily driver but i am having a tiny electrical problem wich i think is either the starter or some thing i dont know about alternator has been tested and is running right while i was driving her the other day it shook a little as in a hint of wanting to turn of then it did after that i was only able to switch it on and make my turn when it shut off again it seemed that the battery was dead but it wasnt truck wasnt taking in any power the battery was fine it wouldnt crank for crap now ive also noticed that my headlights and cab lights are very low untill i throttle it ive installed a powermaster starter and had to insert a diode to make up for the R terminal could it be the starter that is defective or is it that the alternator cant supply enough power back to the battery? some one help please!!!!