Topic on Talk:Dark Souls: Prepare to Die Edition
Falling through the floor at higher than 30FPS is an old and well known issue, search for "ladder" in the comments here http://blog.metaclassofnil.com/?p=232
I can't elaborate much, the issue is probably just "collision detection" which fails in different ways at different framerates. I do not encounter the ladder issue enough to properly give you the highest "bug free" framerate; somewhere above 30 is all that is certain. I could find out exactly when the slow-running bug manifests. The problem is I know it manifests around 80FPS, and when you're that close to 60fPS you might as well just use 60FPS...
Re: Limited amount of bugs- up to 120FPS there are for sure two known bugs. I don't know if more will manifest at 600FPS.