I have found a way to fix those irritating 'snow' effects while using the i7 4790K's integrated HD 4600 (I have not tried with other processors, so it may not happen if you are not using a 4790K).
1. A working El Capitan install using Clover.
Has anyone managed to solve what I think is a resolution change during the boot process to Yosemite? mid way through the loading process from the apple logo, the screen turns black then turns back on and I can see the login screen, (much like when a game or other software changes the screen...