I recently installed 18.04 on my Dell XPS 15 9560. When I use the built-in display, everything is fine; smooth as butter. As soon as I connect my external display (LG ultrawide), I get a 500ms hiccup about every 3 seconds, and it's really annoying. The pointer doesn't have it, but it's very noticeable while dragging windows around. When the external display is connected, the hiccups also happen on the built-in display in a dual display setup. Rebooting is the only solution fix it on the built-in display, But they always remain on the external display.
I didn't have that problem on 17.10. Looking at the system monitor I can't see any patterns relating to the hiccups. What can I do to locate the problem?
In the meantime I tried with various other HDMI cables and monitors, and the problem persists everywhere as soon as I connect an HDMI cable.
It is not related to Nvidia or Intel graphics. I tried switching but in both configurations, the hiccups persist.
It's clearly visible in the following GIF. Every three seconds UI rendering is halted: