(Electrical) 1986 Chevy cavilear cs sadan 1st gen

[thesimplestoner]

My car has been fine until about 4 or 5 months now. I went to a job site that was a good distance away and on my way back my car was acting funny stereo cutting out lights dimming motor surging. And when I got to my destination I turned it off and when I had to leave no start so I went to Walmart real quick and replaced the battery then boom perfectly fine for about a month and a half an began again so I replaced the alternator and was agai fine for a while then again now it's not but I cn get it jumped and it will start and alfter it starts a may die i a few miles to a week then same thing I got the battery and alternator tested at Napa and there fine but he said when alot of amps are drawn that's where the volts go down please help me diagnose this

 

[zora]

what is the voltage at the battery with the engine running at idle?
what is the voltage at the battery with the engine off?
did you check the ground connection at the frame?

 

[thesimplestoner]

At idle no load around 14 at idle under load it drops (I've seen it as load as 9v trying to run) and when the car is off the battery is in the upper 12s unless my car dies while in motion then its about 11 when off. Some one said its my battery but its not because any battery I put in I have same result and as for the alternator I have replaced it because of this issue but did not help

 

[zora]

It's supposed to always be in the mid 13's to mid 14's when running. When off a fully charged battery should be 12.75. If it drops to 9v while running you have an alternator problem. Check the wire connections at the plugs and move them while the engine is running with the voltmeter connected to make sure you dont have a broken wire hidden inside the insulation.