XPS 8930, constant performance issues

I have had my XPS8930 for less than a year but have experienced constant performance issues. Slow to start, freezes on many occasions, displays poor picture quality when first streaming video, unnervingly displays its on light even when it’s been shut down, and so on. All of this despite running Support Assist regularly, and following the ‘fixes’ suggested by Dell which appear to work for a short while, then the performance turns bad again. Any suggestions? I’m within the 1 year warranty period but am really at the limits of my patience.