Keyboard settings don't stick or don't work

[MR ZenWiz]

I'm running Xubuntu 14.04.4 with xfwm4 v 4.11.1.

I recently installed this from the Xubuntu ISO fresh and have noticed
some strange issues with the Windows Manager keyboard bindings - they
either don't stick or they just don't work.

I prefer to use a different set of key combinations for moving between
workspaces and moving windows between workspaces from the ones that
come with the default installation.  If for no other reason, <ctl><F4>
means to close a tab in most programs, but this shifts me to ws4 in
this installation.

So I changed them to ones I prefer.  These work for a while, but after
some apparently random time period, and usually after any reboot, the
bindings are lost or just don't work.

E.g., I have them set now to use <super><number-pad-#> to switch to
workspace #, and the settings shows as such in the manager window, but
I just rebooted and the keys don't work - I have to use the <ctl><F#>
keys to move to that workspace.

Where is the file that contains these settings and how can I make them
stick so I don't have to reset them on every reboot and after random
intervals when they stop working?

Thanks.
MR