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Trig are the indicators the single wire type or the twin wire type? Have you tried it with different indicators just to see if the fault lies with the indicators. Not sure why but I'm thinking earth fault here. Thinking that the vibration is making the earth lose and gain contact so fast the bulb is flashing as fast as a result. This would mainly be if the indicators were the single wire type and relied on the earth at the junction between the frame and indicator, even 2 wired types could be affected the same way if the connection in the indicator was loose.
I'm sure I'v said it before but I will say it again My 250u indicators corroded inside causing them to stay on. Mick
You may have masked the underlying problem but if it works for you then that's great. An electronic unit will probably have its own voltage regulator inside, which will be insensitive to any battery voltage/charging excessive fluctuations