You can disable it in several ways, depends on how your confi files are set.
All ways implies editing config files.
Maybe in you apache config file (normally called httpd.conf) there is a include directive with a file for all the pagespeed config, the you need to comment these line with a # at start fo like.
Maybe you have the pagespeed configuration in the httpd.conf directly, so you need to comment all lines of the pagespeed directives or find one thar say "ModPagespeed on" you can change the "on" with "unplugged"
Or maybe the pagespeed is enabled in you .htaccess file....
Anyway if you can locate all instances in the config files of
"ModPagespeed on " and change it to " ModPagespeed unplugged" this disables pagespeed.
But my bet is the FOUC is not caused by pagespeed module. FOUC is caused by loading css files too late and pagespeed module don´t move css files or snipets, pagespped rewrites these files/snipets in the same location the original files/snipets are.
Can you share a url to test?