My 03 silverado 5.3l has had some issues the last 3 days. It started with my volt meter spiking at 19 v and lasting a day then going back to normal and this has happened 2 times. Then yesterday while sitting at a red light the rpms started fluctuating between 500 ans 1000 and when it hit 1000 the truck would start idling a little rough then it would go back down to 500 and feel like it wants to die. While the rpms are doing this the temp gauge is also fluctuating between 0 and 210. This all stops when I take off from the light and does it again at the next stop. I tested the batt and alternater and both tested good. Any suggestions????? I'm frickin stuck.

 

Welcome to the forum...
I would suspect the battery and the alternator also, when you said checked them, was it a load test or just voltage check?
How old is the battery?
I would check the battery ground cable and the ground strap from the body to the engine... check the cable for any corrosion, physically remove the cable and clean.
Check and clean the positive also and at the back of the alternator...

 

Whatever the test they do at orilly auto parts is the test that was preformed. I'll check the ground strap and everything else