intermittent voltage

[Meathammer]

I have an 87 chevy that recently has voltage going up and down. New battery and alternator. Sometimes will run fine at 13 volts then others will drop to 8 or 9. Lights may dim or truck may sputter and die.  Could this be a grounding isdue?

[zieg85]

Grounds bad or internal corrosion on the battery cable.  Are they original?  Any signs of green powdery stuff?

[bd]

This ^^^^^ and check for loose cable/wire connections at the battery, starter and firewall junction block above the back of the engine.  Find the two fusible links at the starter and one at the firewall junction block and give them a decisive tug to see if any of them stretch or separate. 

[Meathammer]

I've tried everything I know to do and can't seem to find the source. I would think it a grounding issue because it started with no changes made to truck. Just hate to start replacing cables without knowing the culprit. Would a starter going bad cause these issues?

[zieg85]

Take a picture and post it of the battery cables. 

[bd]

I've tried everything I know to do and can't seem to find the source....

Did you do this...

...check for loose cable/wire connections at the battery, starter and firewall junction block above the back of the engine.  Find the two fusible links at the starter and one at the firewall junction block and give them a decisive tug to see if any of them stretch or separate. 

Have you performed any voltage drop tests? 
Post the pic that Zieg requested and describe what you've done to diagnose the problem.  Be specific.

[Meathammer]

I didn't see where anyone requested a pic. So far I've checked for corroded and loose wires and also tightened the bolt on starter that was a little loose. I don't have a voltmeter yet so haven't done that