There are many things that might be going 'wrong' - first off is draw in. Cottolin is a fairly 'weak' yarn that can't tolerate much draw in or the stress of the weft if the shuttle bobbin is really full.
Linen is a very dense yarn and a very full bobbin can create too much stress as it is being seated around the outside end.
How well are the bobbins wound? Are they feeding the weft off smoothly? (I'm assuming a boat shuttle.) If there are peaks and valleys in the winding, the weft can catch on the high spots. Or the linen can leap off the bobbin, especially if wound higher than the flanges, and wrap around the spindle of the shuttle. The sudden jerk as the bobbin jams in the shuttle will stress the outside selvedge threads and could eventually cause them to break.
In an arid climate, getting the linen into a moist environment will help make it less brittle, less stiff and more pliable. I worked out how to make a humidor for my linen bobbins - on my blog http://laurasloom.blogspot.com I think there is a 'linen' label. Or I think the title was Humidifier Bodge which should come up in the search box.
If shuttle handling and bobbin winding isn't the problem, I'd look at draw in. A temple might help in that case. People make temples out of tarp clamps and weights - I'm sure a quick search on the internet will bring up resourses.
Since you have used this yarn previously and had no issues with it I'm assuming the warp has been wound well and there are no issues stemming from that aspect.
If the yarn is just too weak, it can be strengthened by spraying with spray starch, rubbing the outside ends with beeswax or applying a sizing. All of these can be removed during wet finishing.
Hope you find out what is happening. So frustrating. :(
I have a video clip on You Tube showing a small hand mangle. And I have a whole dvd on wet finishing available via Craft Daily (best price) which includes info on cold mangling vs hard pressing vs ironing.
And I love, love, love Kerstin's cold mangle. Unfortunately the commute is a killer. :(
the 'it isn't finished until it's wet finished' person... :)
I do find back hinged treadles a little more difficult. Does your loom have a treadle 'gate'? This can help keep the treadles from wobbling around so much. I think Su Butler might have photos on her website but a 'gate' is essentially a piece of wood with dowels inserted and then laid on the floor so that the dowels prevent the side to side wobble of the treadles. If you then have a foot rest and can tie the treadles up so that you can rest yone heel on the foot rest while the other treadles it can be a bit easier to move your feet from treadle to treadle. Some sort of tactile marker on the foot rest can help you find where you want your foot to be.
Changing the tie up to better flow might also help. Tie ups are not written in stone - as long as you use the correct combination in the correct sequence, you can tie the treadles up any way you like. :)
who apologizes for the multiple posts...if I knew how to remove them, I would!