Your code is the most recent one, the code that I posted is the immediate change by r6191.
What EXACTLY are you doing when the grid flickers? What does "drag" mean to you? Drag the scrollbars? Drag a cell at its border? Drag the form size? Drag the mouse with button pressed to increase/decrease the selection rectangle? Just move the mouse over the grid, no mouse button pressed?
My computer only uses the built-in graphics processor, no dedicated graphics card. So, it certainly is not the fastest.