the forum has been experiencing 'brownouts' recently
There are a bunch of selectable features within vBulletin that cause issues as the size of the database grows...most of these include the following warning in the Admin Panel "This can have an effect on performance."
Forum Jump is one of those. It was switched off a few years back when it was an issue. At that time, hardware changes and some script tweaks were made. Not sure that will be an option this time.
Simply, the issues are sporadic and have been impossible to pinpoint. If you've ever experienced a battery drain issue, you know the frustration...so, we are tracking the same way: pull a 'fuse' and see if the issues go away. If they do, then it's time to decide on options.
The PM system has been helped by reducing the max PM limits and with some other script tweaks. This was simply next on the list, since the issue persists.