300k+vortec

My 02' 5.3L was running fine with the exception of the volt needle bouncing for about a month. I stopped to get gas and then truck wouldn't start. Dead. A kind soul gave me a jump start. Took a while but finally charged enough to start truck and I drove the 30 miles home. Shut truck off and tried to restart but dead. Had battery tested and it was bad. Got new battery. Had alternator checked and it passed. Put both in truck and it started right up. I thought problem solved but seems like it is idling slower (like it is wanting to die) and volt needle still moving some. Bit of a lag in trying to rev it up and about dies when let back off gas. It will die if lights are turned on while idling. Engine just slows and dies. All terminals and connections have been cleaned.
Any ideas what might be going on?