1989 GMC Truck Wont Start! Still

I put a new cap and rotor because they were croded and I pulled the engine block ground off and it was tight and clean. It will start if you jump it off, when you let it run for a while and then turn it off it will start back up great. But after I start it and turn it off a few times it will click and sometimes it will turn over if you hold the ignition to the start position. Seems like the battery is losing power, but I have cleaned the battery cables and starter and altanator terminals.