When the turn signals blink erratically (many times faster than normal), this is normally caused by a worn bulb...the relay has to power up two bulbs and will blink faster when it can only power one bulb. I presume that your bulb is not worn, otherwise you would have seen the control light on the dash illuminated as saying that one of the bulbs was worn.
The relay that powers the turn signal is a combi relay that powers the rear screen defroster and windscreen wiper as well.

I have seen C classes with a turn signal problem that occurred occasionally and every time the problem (no turn signal) occurred, the control light of the rear screen defroster in the climate display was illuminated.
There was a short in the combi relay and after this relay was changed, the problem was solved.

You say that you had a dead battery once and not more...
I don't know whether the combi relay does cause your problem, but I cannot think of any other cause of erratically blinking indicator bulbs (when the bulb is not worn)...

just a thought, greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)