Question: I have a 2015 Dell XPS (new style), and sometimes it exhibits the above behaviour. Typically once a week or so. I've not noticed any common pattern that triggers it.

A reboot always fixes it, but is seldom convenient!

I've tried disabling/reenabling the touchpad in decive manager etc, to no avail.

Any ideas?

Answer: 
Go to control panel and uninstall the driver for the touchpad. Go to Dell's site and download the driver for your model laptop. You can open the program for the touchpad and modify the settings for it. It could be that another piece of software or service is interfering. Try doing a clean boot of your pc - as seen here <https://support.microsoft.com/en-us/kb/929135>. Once you do that go, start enabling different startup items until you find the culprit.
This no longer seems to be a problem (after being plagued by it for a couple of years), so I assume Dell or Microsoft finally fixed something. If you're having the issue make sure everything from Dell and Windows is up to date and if you're lucky it might fix things for you.