Have you possible made your php.ini file recursive on your account? If you haven't done this already, you should be able to create a php.ini file specifying the directory that PHP should use to place temporary files on the server with this setting:
That way the server will utilize your own home directory's /tmp directory, instead of the server-wide one when doing things like saving WordPress plugins for installation from your dashboard.
If you're still having any issues at all, please let us know!
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