Further Diagnosis - Charging System

[Zilverado]

So here's the deal:

After driving a little (engine warmed up), come to a stop light, stay in drive, the following happens:

Charging system volt meter drops from 14 to approx. 12.5 V.
Engine rpm drop slightly, and idle becomes slightly rough.
Dash lights dim, fan slows, etc.

So the question is, does the drop in the charging cause the engine to slow or is the engine idle effecting the charging? You know - the chicken or the egg thing???