99 Suburban Electrical power surges

I have a 1999 Chevrolet Suburban 4X4 with the vortec 350. I have replaced the heads, battery's Distributor, fuel injectors, and now I have my power accessories going in and out.
What I mean is, If I have nothing on My alternator gauge reads in between 9 and 14. If I turn the rear fan on I can see it decrease closer to the 9. The gauge will bounce a round with turn signals on and every once in a while I will hear the fan take off and the gauge will jump above the 14 mark momentarily then back down. This affects anything I turn on electrical. After driving for a few miles it will turn my radio on and off and most of the dash bulbs will blink on and off.
I have a dual battery hook ups and both batteries are new. I had the alternator checked at AutoZone after the service department replaced one of my batteries because he said it was bad. AutoZone says the alternator is good. I have checked the cables on the battery and they seem fine. I have also checked the grounds on both batteries and they seem ok. And I don’t see any corrosion. Any help would be appreciated.

you I'M sure will find a poor ground...the engine grounds must be removed cleaned and secured...cables/lugs at these points must be soldered,crimped, and covered with a heat shrink insulation..

since you added another battery I suspect this is whats wrong..

on auto zone ok I can't say with all certainty that the person that checked your alternator is incompetent...but if he had knowledge to properly test your alternator do you think he would be working there,,,,,

so then it would be to test the current output of the alternator..with all devices operating to create max demand....

You where correct I found one of my batteries ground cable and came loose from the block. I hooked it back up and now it is not jumping around. I just posted another thread witht he following "I have a 1999 Suburban 4X4 that the dash gauge for the charging system show half wat between the 9 and the 14. I checked the voltage at the battery with the car running and it said 14.1 volts DC. If I turn on all the accessories the needle shows in the red and I get a chek gauges light on the dash. I then checked the battery voltage and got a 14.0. Could there be something wrong with the alternator is could this be a gauge problem? " I have taking it to walmart and autozone they didnt agree one said the alternator was fine the other said is was bad.

You where correct I found one of my batteries ground cable and came loose from the block. I hooked it back up and now it is not jumping around. I just posted another thread witht he following "I have a 1999 Suburban 4X4 that the dash gauge for the charging system show half wat between the 9 and the 14. I checked the voltage at the battery with the car running and it said 14.1 volts DC. If I turn on all the accessories the needle shows in the red and I get a chek gauges light on the dash. I then checked the battery voltage and got a 14.0. Could there be something wrong with the alternator is could this be a gauge problem? " I have taking it to walmart and autozone they didnt agree one said the alternator was fine the other said is was bad.

the only way to check the alternator correctly is to measure the current output...

I use a probe the clamps loosely around the red output wire...this then produces a MV output that then goes to a voltmeter..this is then displaying the current flow amount...a very simple and fast way to check the alternator current..

my vehicle 2000 5.3L 4X4 with all electrical loads on causes the alternator to output 66amps...