If the CPU cycles are reduced to 1, the bottom of the screen is no longer updated. Unlikely to ever happen in real-world situations, but a bit strange. It DOES work OK at 2 CPU cycles...
Interesting bug, but as said not really like to happen in real world situations.
Not entirely sure what it happends though.
70 hz with 4 screenparts gives me 280 events per second. 1 cycle is 1000 steps in emulation. So there is room to do them.