I am having a problem with my menthol, I'm mixing menthol crystals with PG (crushing crystals,mixing with PG and heating up in microwave for 5 seconds) then pouring that mix into the pre made menthol (cuz I like it strong) Anyway if the mix sits for awhile the mix forms a crystal crust in the bottle(clogs up opening in the bottle) until I warm it up(I thought heat was an enemy of the nics shelf life). Is this normal? I want an EZ to use 24 mg, strong menthol,what can I do??