Do as Chris said Gab, attach the screenshot to your first post.
I can reproduce, same thing happens here.
I have tested on three workstations (MacMini/MacPro/OSX/Win7-32+64Bit, with Cubase 5.53, Cubase 6.05, Nuendo 5.5)
sorry, cannot reproduce, all working fine here.
but…on my MiniMac I have a curiosity: only very rarely a diamond is represented half. which disappears when I move the cursor or click to zoom.
so…I really carefully think it’s a hardware problem with the graphic card (?)
but on your two systems…? hm…I don´t know.
strange.