[xubuntu] / filled up then 'no space left' and lockup.

Hi all. Is this normal for the contents of these folders directly after boot?

$ dir /var/tmp
systemd-private-5865b17aff664ec69d18a2659e6b07f6-colord.service-X50gy3
systemd-private-5865b17aff664ec69d18a2659e6b07f6-rtkit-daemon.service-PEEa3z
systemd-private-5865b17aff664ec69d18a2659e6b07f6-systemd-timesyncd.service-5rNeMz

$ dir /tmp
config-err-HdRP7Y
systemd-private-5865b17aff664ec69d18a2659e6b07f6-colord.service-axGfFO
systemd-private-5865b17aff664ec69d18a2659e6b07f6-rtkit-daemon.service-osktJd
systemd-private-5865b17aff664ec69d18a2659e6b07f6-systemd-timesyncd.service-4V2rtl

After that, no space left on disk messages every time I tried to launch anything. I couldn't open gparted to see if the disk actually was full.

Anyhow, I'm on 15.10 with 16.04 on another partition so booted into 16.04 and, sure enough, the 15.04 / partition had about 700mb left available and was full. Crazy thing is, there is virtually nothing on that partition. So I mounted it in a file manager and started to hunt. There is nothing anywhere that is anywhere near big enough to fill 20Gb (the size of the 15.10 / partition). The only thing of consequence was the 2Gb Uni folder which contains all my work. I backed that up, deleted it from the / partition and rebooted into 15.10 which now had enough headroom to allow me to dig a bit deeper.

I can still find nothing, but the output at the begiing of this post might give a clue. What I did discover was that both those folders had about fifty lines like it and a few other bits and pieces. Where have they come from? Deleted the lot, rebooted, and without doing anything else, I get what is shown in my output above in those folders.
_____

Hunted and grunted (to put it mildly) and found a bunch of clues about what might be happening, a link is still open for a deleted linked file looking promising, then there's this and this, none of which has done anything for me (although they might if I had any idea what to do with the output ... clutching at straws there).

I've always had issues with USB on this machine (new build, i7, 16Gb RAM, NVidia GTX 970, Gigabyte MB) where the keyboard dies and comes back to life randomly and less often the USB network adapter does the same, so wondering if related.

Frankly, fairly clueless with what to do at this point and deep breathing (ain't deadlines fun). Have found a workaround (mount 16.04 partition in 15.10 to access the backed up Uni folder stored there) but it is pretty important I get to the bottom of this. And yes, there is nothing in trash (although something about hidden trash was in some thread I read). About to get back to work and half expecting the /var/tmp and /tmp folders to start filling up when I do. Is there anything I should report? Any log that would help folk help me diagnose?

For now I'll get back to what I need to be doing via my workaround (after six lost, valuable hours screwing around with this) and check back later. Appreciate any and all clues, I have all digits and eyes crossed, and TIA.
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*** NOTE: I just booted in to 16.04 and before doing anything this is what I have in those same folders here:

$ dir /tmp
config-err-fPbv94
systemd-private-eff1f475153245fd9f05e62f79fcda58-rtkit-daemon.service-QbtlvI
systemd-private-eff1f475153245fd9f05e62f79fcda58-systemd-timesyncd.service-MBo1W2

$ dir /var/tmp
audacity-bucky
systemd-private-4db87bd99aec489eae4f4550e794d9fa-rtkit-daemon.service-QO1cAl
systemd-private-4db87bd99aec489eae4f4550e794d9fa-systemd-timesyncd.service-6d3NEe
systemd-private-8aafe2f854634f41ab28bde4f7d52fda-systemd-timesyncd.service-dF9DyW
systemd-private-b688b69e31e442df987172be2aac0be0-rtkit-daemon.service-oq7gsR
systemd-private-b688b69e31e442df987172be2aac0be0-systemd-timesyncd.service-Gvyxxb
systemd-private-eff1f475153245fd9f05e62f79fcda58-rtkit-daemon.service-z8MDtC
systemd-private-eff1f475153245fd9f05e62f79fcda58-systemd-timesyncd.service-fDZY6S

And just for good measure, have attached a pic of htop from 15.10 with no apps launched on desktop. I see some suspiciousness there (sbin action?) but no idea what to do to dig further.