Voltage gauge needle fluctuates ??? Any ideas?

On my 94 5.0 5-speed, the needle on the voltage gauge will fluctuate left to right. It does it on a regular basis, but occassionally is solid in one place. Also, there have been times where under acceleration, the needle will take a big dip to the left, then back to it's normal operating location.

I've replaced the battery and the alternator/voltage regulator, and still the same thing......

Any ideas?

Thanks in advance

Kelly