Guys, I have a Stubby 18650 X-ray (NOT DNA), and I had the same problem. (at 50% battery the mod says "low battery", low voltage and wattage gap after 80%.
I also have the DNA splatter edition and I heard that many people have the same issue even on this model.
After I cleaned everything, tightened every screw, (spanned many and drilled to remove them ), I changed the board and nothing!
I don't know were I found the strength to keep on trying but finally I understood the reason of the problem!
The problem is that on some items after some months of vaping the roller upside the negative side of the battery need to be tightened more and more.
I had thought about that but I was worried to ruin the mod (breaking the up plate).
The reality is that the battery need to be very grasped in the slot (don't stop tightening until the roll is completely blocked! if you don't have the nails use something od plastic to spin more strong).
My mod was terrible, it didn't fire after 70%! and the power was 20W when I was putting 40W !
I have also sanded al the contacts of the board and the plate, but the situation changed completely after I tightened more and more the battery's roll.
Vape on!