Hi Salsaman,
Can you be a bit more specific? then I'll try to help you.
- A schematic is easier to read than the board file
- What are you pwming? Big LED's? Motors?
- Are the shift registers running at 5v? (big mosfets often need a bit more (a small mosfet to switch the bigger mosfet).
It seems you have found a bug, thanks!
Up to 127Hz, 256 levels, 6 shift registers its all good. 128 Hz, 256 levels -> crap.
When the interrupt frequency exceeds 32768 (max of int), a variable overflows and the interrupt frequency is set to a way to low value.
I am sleepy now, more tomorrow!