Screen Blacks out while zooming horizontally

Hello there,

i’m having a weird issue on multiple Systems in Cubase Pro 8 and Nuendo 6.5.

While zooming in and out horizontally, the Nuendo/Cubase Window blacks out irregularly - sometimes for a few milliseconds, sometimes for a whole second.

Its not the whole screen that blacks out, “only” the arrangement window and the project overview.

Both machines are Mid 2011 27" iMacs.

One is running OSX 10.10.5, Nuendo 6.5.40 Build 315 (64bit),
the other OSX 10.10.5, Cubase 8.0.30 Build 536 (64bit).

Both machines have 12gb of RAM and an AMD Radeon HD 6970M 1024 MB Graphics card.

I have attached a video that shows the problem.
Does anyone else have this issue? It’s starting to really slow down our workflow. :frowning:

Thanks in advance

The attachments did not work, here’s a WeTransfer Link with the video: