Performance problems in editor under specific versions on Mac

Hi,
i am using Unreal Editor version 4.7.6 on Mac and the editor lags all the time.
It doesn’t matter if i make a new project or use an existing project, the editor lags.

When i was using version 4.7.3, it was also extremely laggy, but with 4.7.4 and 4.7.5 i had no problems.

I also downloaded the preview of 4.8, this version works smooth.

How it is possible that some versions work smooth and some versions are unusable?
I want to use the Unreal Engine for my final school project, but if it doesn’t work stable i can’t use it.

My machine is an Macbook Pro 15" 2011 with an Intel i7 2.2GHz Quadcore, 8GB RAM and an AMD Radeon 6750M with 1GB GDDR5.

Thanks for any help.

Please, i need urgent advise!