If using Frozen Columns with MultiHeaders, where one column has many headers while the other fewer. Each header in a column occupies some space and they get hidden, this will cause the upper (header) portion of the Grid to change its size. With that, the scrollbar will move away from the mouse position and causes the scrolling jump back and forth resulting in flickering.
Demo video of the issue: RadGrid FrozenColumns Flickering Header. The video can be found in the attachments as well.