I'm probably mentioning some things that you already know, but here goes anyway....random problems are tough to find sometimes....
check the sheilding gas lines/fittings for leaks, including the torch?
check gas flow pressures?... too low not enough coverage or too high causing turbulance
contamination on materials, oil, paint, coatings..be sure the material prep equipment hasn't been used on something greasy ot has paint lodged in the grit on the grinding disc...ect...?
contamination of filler?
shielding gas itself?