I use a few different makes of stock coils for both MTL and DL and on about half of them I get a temporary slightly bitter taste when they're brand new, then after a tankful it's fine. It seems to happen less on meshes. What is the cause and can I do anything about it? I've tried breaking them in gently, gradually increasing the power. I've tried different rampup/preheat settings. I've tried different PG/VG ratios, nothing seems to make any difference.