Carbon deposits usually. Not neccesarily a function of fuels, can be caused by materials.
You mean the fluctuating flame could be down to carbon deposits? I ask because my latest lighter to do this is an electronic Vertigo. The flame is stable once lit but it changes wildly between lights. You light it, put it down, and pick it up five minutes later and the flame is four inches long or it's disappeared altogether. It's my table lighter so it's not rattling around in my pocket and it's less than a month old.
It could be the design because the flame adjuster wheel is large and fully exposed on the base and turns maybe a bit too easily, so I could be turning it just in picking it up. But now I'm aware of this I'm careful to avoid touching the wheel and it still does it. It's a shame really as it's otherwise a very solid feeling, well built lighter.
The Rattrays, which was twice the price, is the lighter I take to work because it's more dependable. I'd planned it the other way round...
I would like a nice looking reliable lighter to sit on my coffee table.