I too experienced this then found this snippet in the docs, under the Flip command which explains why this happens. Hope this helps :)
However, it is worth noting that this setting applies to the graphics card only, and some graphics cards allow the user to disable the vertical blank event. Therefore, if you rely on Flip alone, the display may be corrupted on certain setups. The VWait command may be useful to you in this respect, as it forces the CPU to wait for the vertical blank (as opposed to the graphics card), and this cannot be disabled. Hence true silky smooth updates are best achieved using "VWait : Flip False".
However, it is worth noting that this setting applies to the graphics card only, and some graphics cards allow the user to disable the vertical blank event. Therefore, if you rely on Flip alone, the display may be corrupted on certain setups. The VWait command may be useful to you in this respect, as it forces the CPU to wait for the vertical blank (as opposed to the graphics card), and this cannot be disabled. Hence true silky smooth updates are best achieved using "VWait : Flip False".