F26 hangs or dies

[Stephen Davies]

I upgraded from F25 to F26 yesterday and ever since have been seeing the 
system frequently become totally unresponsive.

It seems to be quite random and can only be resolved by hitting the reset 
button to reboot.

On other occasions it doesn't quite die but starting anything takes several 
minutes rather than seconds.

Thunderbird and dnf are examples but sometimes there is just no response to 
key strokes or clicks.

The only clue that I have seen is that top often shows very high wait I/O 
levels and swap space is sometimes (but not always) low.

Nothing has changed in the system workload or configuration.

This is a production machine so any help will be very welcome.

Cheers and thanks,
Stephen