Post Reply = 30 seconds of waiting

For some reason it has begun taking aproximately 30seconds to post a ticket reply from our control panel.

The same goes for new ticket creations through the control panel.
At most times it takes 30 seconds, and a "success" message is shown.
Other times say 1/10 it goes to a blank screen instead.

Im unable to determine where the cause lies - but can say that a test mail from Osticket is posted "instantly" without delay.

Any help much appreciated - thanks.