OT probably, help please

[James Linder]

G’day
The other day I reported that gnuplot had suddenly stopped working with term x11 but worked with term aqua.
Maybe it was just slow !
I have an issue where the system has (suddenly) become very slow …

iMac 27 top:cpu free 99 ish %
system-monitor also shows cpu cores at 0% busy
men free 8G
df 50 ish %
yosemite

change mail from one entry to another: wheel turns for 10-20 seconds then all is normal including change topic (repeat previous selection).
firefox:youtube watch a clip, midway stops playing and wheel turns for 5-10 seconds before resuming

Thinking this may be a DNS issue I ran wireshark. wiresharks stops while wheel is turning!

system-monitor shows no (just got the wheel for 5 secs, no response to usb-keyboard nothing interesting on system-monitor) sys-monitor (pause again!) ticks every 5 secs even during wheel turning.

This seems unrelated to macports. Time machine is on an external USB disk and scheduled to run now (another pause checks:TM disk definitly NOT busy)

So before I nuke this install, any ideas gratefully acceped
James