During the first 6 months I do not believe that you have to prove it, the onus is on the retailer to prove that it is not faulty.
Moreover intermittent faults are extremely hard to replicate and prove. Hence "varied and intermittent faults" such as graphical glitches is a handy phrase when returning an item.
That said pictures of the problem won't hurt. However some instore muppet might try to "diagnose" the problem if you give them too much ammunition.
I think it best to simply say it has X fault on an intermittent basis, I would like a relacement or refund as per my statutory rights afforded under the Sale of Goods Act please".