Short explanation: There should be a driver file name in the blue screen, which identifies the driver that is at fault (it will often end in ".sys"). This typically means that the driver was badly written. Google the driver, and figure out which device it belongs to. Then go to the vendor's webpage and check for an update to the device driver. If it's that bad, hopefully you're just running on an older version and they've already fixed the problem.

Long explanation: The most common reason for encountering DRIVER_IRQL_NOT_LESS_OR_EQUAL is that the driver tried to access memory that was paged out while processing an interrupt. Since while processing an interrupt, the driver is running at such a priority level that it even defeats paging requests, doing so would result in the driver not getting the information it was asking for. Since there's almost nothing good to be done at that point - for all the kernel knows, processing that interrupt correctly could be critical to the operation of the system - the kernel proactively takes itself down to protect the machine.

I'm not sure I can think of a case where this is not because the driver at fault was written incorrectly. This can pop up suddenly when the memory footprint on the system changes in any way, typically when more pressure is placed on it. Making an assumption that Photoshop is a memory heavy application, for instance, if you suddenly started running Photoshop on such a system, all of a sudden the memory system is under heavier pressure, it pages out a page belonging to the driver, the driver gets called to process an interrupt, and wham, your system goes down. I think you mentioned this happens under heavier stress - that's why.