Wow. That's not been my experience with Oracal or 3M products. At all. What brand are you using that it only has a 6 month shelf life?
And we too stand it all on end except my 24" rolls of vinyl - they are on a wall rack.
Just my experience is all, I don't have years of testing data to back that up.
My 180c and IJ35 don't sit for very long so I wouldn't know.
Just about everything I get from General Formulations (they make lots of "house" brands) seems to not print as good.
And if you can get static cling to last for more than 3 months...let me know what brand you use and I'll switch. That stuff dies out quick.
I do lots of flat bed printing on coroplast, I know that not what we are talking about here...but it looses its corona treatment over time.
I also seem to have bad luck with just about any brand of clear vinyl.
.... I have seen this "humping" on a roll of 2 sided banner once that wasn't rolled tight.. btw it sat for a long long long time unused until I got my UV printer. Turns out my baby mutohs don't really like 2 sided printing much.