Hi,
I have had a couple of spools of Orvis Mirage weaken and snap under fishy load this last year. Interestingly, both were well within their best before date as well. I did hear that they are not good when exposed to the sunlight for long periods, and both of these spools came back with me from Las Vegas, so could have been kept 'badly' before I bought them. Too much sun is not generally a problem in the UK!
Personally, I would suggest bringing spools back into the house and storing them somewhere in subdued light and seeing how you go. Also it does appear that the more expensive ones do perform better.
I've also discovered that some flourocarban is very different to other makes. Generally Orvis Mirage is good and also Rio, but I have had very mixed success with some other makes. I bought a spool which had plenty of line on, which was sold and designed for carp fishing, to try and lower the cost of tippet material. This stuff was absolutely hopeless, and broke as soon as you looked at it. I'll upset too many manufacturers if I start listing the dodgy ones I've tried over the years.
