It's probably wicking too slowly, especially if when waiting a while between drags it tastes fine. I agree that thick liquid could be causing or exacerbating the problem. More PG and less VG probably would help.
I think the 510cc uses the same atomizer, which in my experience, doesn't easily behave well with high VG content. I go
through a lot of extra trouble to make the eroll work with high VG, but I'm tired of the frustrations and am now going back to high PG.