From http://forums.adobe.com/thread/1436085?tstart=0

The bug is easy to spot, just create a gradient in sRGB and convert to ProPhoto RGB, if Drawing Mode = Normal or Advanced the gradient may get banding, clipping and color casts.

The remedy is to set Drawing Mode = Basic
Go to Menu>Edit>Preferences>Performance>Graphics Processor Settings>Advanced Settings and set the Drawing Mode = Basic