About 2 weeks ago my voltage gauge began bouncing like a spring randomly no specific time or RPM zone. After it fluctuates for a second the fuel and temperature gauge will plummet and sometimes the speedometer will bounce. It has a new alternator battery and starter and no AC unit. I've checked all the connections had everything load tested checked all the fuses and grounds and can't seem to find an issue. I would normally think that it was just a typical GM cluster issue with the stepper motors but when the gauges are going funny I have a slight performance issue the engine feels sluggish almost like I'm driving into a strong wind for a second. I can't for the life of me figure out what'* going on

Is this on the 2001 Silverado 1500 5.3 in your profile?

What does a volt meter say about the battery voltage at the battery terminals while the ignition is off?

What does a volt meter say at the battery terminals while the engine is running at idle?