JJJ is bang on the money, the forum is hitting the buffers of it's allocation; database, bandwidth, whatever;
I think I've mentioned this in the past but I work in IT in server infrastructure - I'm not particularly experienced in PHP - if the current administrator wants to offload it and if I can afford to run it, I'd be open to taking it on as a running concern. Too many great phpbb forums have been lost from the internet, with the treasure trove of information contained within.