Also 85 is not a multiple of 60 so I don't see why only a setting of 60 provides smooth turning..
Are you certain of your display rate? I also took the CLS line out so you could try that (it's best after Flip I tend to think). Also, you are waiting a certain amount of time before drawing each frame but never bothering to make sure that each frame you send is synced to the vertical retrace - this is why distortion occurs. You can either throttle that procedure by while...wending until the appropriate scanline is reached or simply Flip True instead.
Dungeon Siege runs at about 37-47 fps on my system but it never jitters
Probably it vSyncs, leaving only the remainder of the update rate into the monitor refresh rate to cause visual quirks... it's harder to spot these in a full, textured 3D scene - especially when the camera is in motion, especially when the view is being rotated rather than panned. Were Dungeon Siege a side-scrolling shooter, you would notice the jerking straight away.