wkhtmltopdf works only occationally

I have wkhtmltopdf running on an Ubuntu 11.04 64-bit server. Running it from shell always works, but using it together with Drupal/Print usually results in a perpetual browser state of "Waiting for [mydomain]...". Even worse, for each failed attempt, a wkhtmltopdf process is spawned but never terminated.

At times however, (when the server load is low, I suspect) PDFs are generated beautifully. I guess this is a matter of general server configuration and I'm hoping that someone with more experience than me, might have run into the same problem. Any ideas would be greatly appreciated!