Relax, you’re not going crazy, it probably was on DMA last week.
You didn’t say what operating system you are using, but if it’s WinXP it is well known for automatically and gratuitously disabling DMA. The fix for this is go to Device Manager and uninstall the IDE channel to which the device is attached. Upon rebooting the IDE channel will be reinstalled and hopefully the device will be redetected and installed with DMA enabled.
Moving from master to slave is not affecting the operation of the drive, but it is the redetection of the drive that is causing DMA to be reenabled. You have already proved that by temporarily fixing the problem by switching master to slave and vice versa.
As to your device drivers question, since your gigabyte 6VTXE mainboard has a VIA chipset, you could download and install the latest VIA 4in1 v4.43 Drivers. This usually helps in getting DMA to work properly.
What operating system are you using?