Basically, we have a list of things to work through. We've just removed the possibilty that it might be a not-powerful-enough-hardware issue by increasing the RAM to 1024 MB. Next is to double bandwidth.
If those two measures don't solve it, we'll have narrowed it down to a software issue. We know that the boards were running fine until recently, so if it does turn out to be software, the only supect in the running is PostNuke - we'll be looking carefully at options (downgrading PN to an older, but proven version, upgrading to a newer version etc.)
If it's not hardware, bandwidth or software, then it's an Act of God and beyond our mortal powers to solve. But, logically, it will be one of those three things, and we've just eliminated the first.