I think that is a good result, in that you have tried everything obvious and narrowed things down nicely. With LCD, they will almost certainly do a swap out and that will give you a new (my Iiyama was, anyway) or serviced screen that will be fault free. Usually takes about 2/3 days.
Let us know how you go on
Conan
ps don't muck around with the retaqiler unless you have to. They will only delay or muddy the water. Go straight to the manufacturer if you can.