New,
It is easiest to just replace the suspect bulb with a new one.... Ususally the bulb looks fine, but the filament may have a hair-line crack that will cause it to work sometimes and not work others.

Rest-assured, when the indicator blinks fast, one of your bulbs is not working..

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William
76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles