You probably have a bulb out somewhere...
Any variance in the voltage, either increase or decrease (burnt bulb or shorted circuit), will cause this. The most common cause is a burnt out bulb somewhere in the lighting system.
Less common causes are other interruptions in the circuit, such as loose wires\connections or building rust\gunk in the bulb recepticles...
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"The Yellowjacket"
1998 Mustang GT
13.97@103
"Traction Impaired" or "Nitto Needy"
"They think they can make fuel from horse manure.... Now, I don't know if your car will be able to get 30 miles to the gallon, but it's sure gonna put a stop to siphoning."
- Billie Holliday