Spastic voltmeter (factory gauge)

[MountainGoat1966]

83 C20 factory gauges.
When I bought the truck almost 2 years ago, the voltmeter showed about 14 volts charge rate.
Ever since then, it indicates less and less voltage, and now it is dropping below the 12 mark.
Speeding up the engine does not increase by much, it increases to 12 and goes no higher.
With engine running and no other electrical loads applied on my doing, the needle varies in its reading.
A hand held voltmeter on the battery terminals with engine running shows a solid 13.8 so I know the alternator is charging.
I would like an accurate gauge reading all the time.
Has anybody encountered this issue, or should I write it off to the space aliens messing with me again ?

[ratskin]

Always check the ground first.