Excellent colour crisp picture, sometimes play up a little trying to watch Netfilx, but usually a restart will fix

I bought this when it first came out brand new and it was really great, but recently, I've found that 4k and UHD contents don't come out as crystal clear anymore even though I'm paying for Netflix premium just to take advantage of the tv, I have done a reset and recalibrated the picture settings but still without success and there are occasional flickering on the screen when playing Xbox games. Not sure if the TV is still covered by warranty, but it gets annoying sometimes with a lot of picture noise when watching 4k contents plus it's now very slow.