Well, that's going to be a byproduct of the level of hosting that they provide. Companies like Dreamhost and Globat oversell to a very large degree. This means you'll be stuck on the server with hundreds (or even sometimes thousands) of people who all think they have the same amount of resources that you have. This can have a major influence on page load times and errors. Quite often, you can examine the fine print in the TOS/AUP policies and come across the clause where they limit what percentage of server resources you can use. I'm not trying to bad mouth another host because each host has its own specific high and low points, but this is a reality.
My advice is to lower your expectations and shoot for better quality. Forums are not quite the bandwidth and space hogs that people make them out to be. My advice is to go somewhere that you can move up the totem poll so to speak. There are plenty of providers here at NP that you'd be happy with, or you can do your research elsewhere.