Fix Wordpress site - 404 on plugins.php & update-core.php pages

We have problems with the [login to view URL] & [login to view URL] pages, they both give 404 errors, after trying to load for 30 secs or so. I also get errors when installing a new plugin via the Wordpress admin and when trying to update a plugin. Ive tried all the normal fixes: -removing the hataccess file -reuploading all of wordpress -checking permissions on all files -changing php versions -changing themes -adding lines of code to the htaccess file such as: RewriteEngine On RewriteRule .* - [E=noabort:1] RewriteRule .* - [E=noconntimeout:1] If I remove all the plugins from the plugin folder its fixed, I tried then slowly renabling them but it is not obvious which plugin is the cause, it seems to be intermittent. I have about 50 plugins, I need all of them for the ecommerce site. Its running on a dedicated server with lots of ram and a fast CPU. I asked the host, they have been no help, they just said it might be the htaccess file. I have made a staged version of the site and the person I choose to do the work can use the staged site to play around and find a fix. So, plugins page, core update page and the ability to update plugins via the plugins page are the 3 things I want fixed. They probably all have the same cause.