No it's not the slot......

OP download GPU-Z on your PC to detect the type of memory that those GPUs are using, this is the only way we can tell you the right settings for each, also download Msi Afterburner,

Open Msi Afterburner and downclock just like what you see in the image below



Power limit = 65%
Core Clock = -502
Memory Clock = -502
Fan speed = 57

This will drastically give you 30MH to 31MH per second on the graphic cards

Make sure your RAM is up to 4 gig size