I am having issues with fusing on A3 media of 100 - 105 gsm on long runs, where the toner peels off the paper and leaves repeating marks.

Normally this problem occurs only with colour copy paper - that means that the surface of that paper is smoother and so it´s not so easy for the toner (by the normal temperature) to fix 'in' the paper, so the result is ==> toner peels off the paper and leaves repeatng marks.

The specification on the datasheet 60 - 105 gsm / m2 refers to a normal (plain)paper, but if you use a colour copy paper or any other special layer by this weight you will be in a border area. Your only choice is to handle it via media settings or change the paper.