The solution is to this black screen issue is
Dgvoodoo2.
It's a fairly common issue with some modern video cards; possibly only with certain driver versions, as when i used to have this issue it seemed to come and go with driver updates. The reason you're running into it more frequently on certain characters is because the problem only happens in certain areas in-game.
As for the UAC thing mentioned by mccoy, a UAC prompt is the pop-up you get when installing a program asking for permission. If you get one of these while playing P99, then P99 will crash unless you change the following windows setting:
Control Panel > Security and Maintenance > Change User Account Control Settings > Change to second lowest "Notify me only when apps try to make changes to my computer (do not dim my desktop)"