wkhtmltopdf unreliable on my server

I've been trying out the excellent print module and I'm using the latest version v12.2.1 of wkhtmltopdf to allow creation of pdfs.

The issue I have is that 70% of the time it works perfectly and for the other 30% pdf generation process just hangs until it times out and returns the error 'unable to generate pdf' and the log contains

wkhtmltopdf ["/usr/local/bin/wkhtmltopdf" --page-size A4 --orientation portrait --dpi 96 --disable-local-file-access --footer-font-size 7 --footer-right '[page]' --username test --password xxxxxxxx - -] (returned 9): No stderr output available.

The Drupal site has an Omega based theme however the issue also exists when using wkhtmltopdf from the command line for other sites such as the bbc, although possibly not as often.

The pdfs being produced are relatively small at between 40kb and 70kb and consist of 2 or 3 pages of mostly text.

If I look at process manager when it fails it just leaves a process hanging using minimal memory and cpu which stays until I kill it.

Does anyone have any suggestions of anything to try changing on my server or if there is anyway to work out at what point it's failing or what the cause of it is?