Is it this one?
If so the first thing to note is that you are over driving that chip. You can't have 1.2A being switched on at the same time due to the thermal dissipation limit of the chip. See :- Power Examples
Next flickering covers a wide variety of behavior, can you be more specific?
To eliminate a software problem, write a sketch that doesn't read anything from the input and just changes the intensity of each lamp on an algorithm of some sort. Like incrementing one light every 100mS, it's OK to use delay for testing.