Hi Zibby
If the error means that the stock is too heavy, it could also possibly be the platten is a little gummy/sticky from self adhesive glue.
Couple of months ago I had a repeating error "x-over current avg" when i was cutting small labels out of vinyl. I solved it by cleaning the platten, aluminium and powder coated black area and then chalking it with the slightest bit of chalk dust to lubricate it. (Didnt chalk the aluminum bit tho)
This is probably against what a lot of people on this forum would do as the chalk may end up on the head. But i dont think there was a risk as i used such a little amount.
Cheers
Lee