On very rare occasions, the SRC4392 will keep the SDA line low for no reason and the I2C bus is stuck. I do 2 transactions on the I2C bus every 500ms and after either a few hours up to a full day, the SRC4392 will do this problem, the SDA line is pulled low by the SRC4392 (yes this is the chip that does pull the I2C line down as I have 100ohms resistors on every slave so I can easily identify the faulty slave).
This is the same problem as reported in another thread that is now closed without and solution...