Problem is that pins are SDA and SCL pins, that are connected with a I2C port expander pcf8574.
I obtain the same problem if i connect an lcd display I2C to arduino...it activate baklight for some seconds..
So the problem has nothing to do with the Arduino, it's the port expander.
my arduino is 2009
Can you tell me if in your opinion MCP23008 would have the same problem?
Have you changed the bootloader? Does the board have the original processor?No everithing is original