Also you might check your gas (argon, right?) because if you've got a bottle of even slightly contaminated argon (it's out there - I return six bottles a year on average) it will weld very hot, resulting in burn through. If your tungsten is staying nice and clean, then of course it's probably not the gas itself but now flow rates and coverage (cup size and shape) would be the next factor?
Agree with JeremyW83 regarding cleaning petroleum free.