Channel form - 2.9.2 - 503 error

We have been running a website with an Artist upload form.

This form allows for a maximum of 5 JPEG images (max 5mb each) and a PDF (max 5mb each) to be submitted alongside a number of text fields. This had been working fine and we had no problems in the past or with test uploads, however this year just prior to the deadline we started getting random disk write error messages, the server however was not under load and there was plenty of space, in the end turning off XSS Filtering to uploaded files seemed to resolve this issue.

However on deadline day we had a surge of visits which has since caused a 503 error every time a application is submitted, regardless of any attachments. I have talked to the hosting and there has been nothing that they say has been over limits but they have increased all our limits and am just waiting on if they can figure out why the 503 is still displaying.

I am also backing up and upgrading to the very latest EE 2.10.1 and have cleared caches several times.

Any tips on how to increase the robust nature of this or whether the Channel Form is just not up to the job of multiple form uploads in short time sessions.

http://www.10dayswinchester.org