Thank you for allowing me to assist you, my name is Joe and I will be happy to assist you with your concern today.
Generally this is related to a bad/poor connection, sometimes can be excessive draw beyond what battery can handle but is usually not the case.
The best way for set up is one cranking battery for motor only and the other a house battery for all electronics ran to a dual battery switch. It's also best to use an buss bar/block with only a single lead to the battery instead of all connecting at the battery.
Flickering could be a loss of power or ground due to loose connections or poor and or bad wiring. For wiring you have to consider length for voltage drop: http://www.bulkwire.com/wireresistance.asp
Another great one is the 12v bible: http://www.amazon.com/12-Volt-Bible-Boats-Miner-Brotherton/dp/0071392335
One way to check your wiring is to connect directly to the battery and see if it still has the problem as well as checking the connector for corrosion or loose.
Note that the battery should be professionally load tested as well.
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