I had my alternator replaced & got a new battery w/ new terminals, had my starter tested and my car still is showing 13ish on a battery/alternator tester and drops to 12ish then jumps around between 12-13 when anything is on. It then started jumping and cutting out after I stopped, backed up, and turned today so I went to have a diagnostic ran and I then replaced the ingnition coil. It ran fine with no change in the voltage for a few hours then started to jump and cut out again. Anyone have a clue what could be going on? It's a 01 Kia Spectra.
Forgot to add this but I have new belts also .

The thought comes to mind that you may have a bad Alt gage. When you say new terminals are you talking about completely new cables? It's possible you have corrosion inside the insulator on one of the cables. I will always replace both the positive and negative cables when I replace the battery. This way I am assured a true connection between the battery and ground. Just a thought.