Check the voltage at the battery when idling. If the voltage does not stay within a few one hundredths of a volt the voltage regulator is suspect.
The "poor man's" test is to shine your headlights on the back garage wall and see if they change in intensity with the engine running. If the v reg is tired you have probably already noticed this.
Symptom/Solution – Bouncing/Floating/Off Tachometer and Random Bulb Burnout.