Long Backup causes Apache to stop responding

When a scheduled full backup is running my server stops responding to HTTP requests (even for localhost) and it seems httpd-apache needs to be restart to correct that.

The backup takes about 1h30m to run and it uses up 1 CPU core to do it's gzip work. But there are other cpu cores (total=8) available so it shouldn't be a problem.

What can be the cause for apache to stop responding (nothing reaches apache log so I can't see any error there)? I understand load is high on the hard drive but still, it should be possible to backup AND use the site at the same time...

Any tips to improve this?