'Connection reset' after ~2mins over SSL

I’ve set up SSL capability on my WordPress admin pages, and it works, but after about two minutes of being logged in over https I click onto another part of the admin pages and run into ‘The connection was reset while the page was loading’ (in Firefox).

I’m able to browse the non-ssl parts of the site, and I can access the https parts, but only for a short period before the error occurs.

Also, if I visit the non-https front page by typing in the bare domain name, then leave and come back some time later, it then tries to force me to get the front page over https, and gives me the connection reset error.

Clearing ‘active login sessions’ in firefox and refreshing the page allows me to log back in, but only for another brief period.

The only modifications to the page I have made are the SSL configuration (with ‘FORCE_SSL_ADMIN’ set to true in wp-config.php), an installation of the CiviCRS plugin, and the wordpress-importer plugin. If it looks like the problem is to do with one of the last two, I’ll take my question there, but I think it’s to do with WordPress itself, possibly a cookie issue?

Grateful for any help.