Some ideas for why sprites would disappear in 2x Speed Mode
Written by Jan-Lieuwe Koopmans
moa@gelrevision.nl 


- You move your sprites using an ISR (interrupt service routine) and that
particular interrupt is disabled by the cpu-speed switch routine (IE_REG
being altered);


- Bit 1 of the LCDC register is somehow set to zero;


- The DMA copy of OAM workram to real OAM ram is not done anymore (that
routine is in HiRam and is being called by the default VBL ISR).


- Your sprite-tiles are not being copied to VRAM in a proper way.



You can check all these easily using No$Gmb (press F10 in debug mode to see
register contents, check HiRam (starting at $FF80) using the debugger, check
Video RAM using F5 in debug mode).