LG ultrawide screen problems with Cubase Artist 8.5

Hello, Im a spanish user of Cubase Artist 8.5. I hope you can help me with this issue:

I was facing spikes with projects having very few tracks and creates a few UCP load in Cubase Artist 8.5

Mac Book pro end of 2.011
O.S 10.11.3
Intel Core I7, 4 cores, 2,4 GHz
16 Gb RAM
AMD Radeon Hd6770M

Motu 828 Mk2

I looked around internet and firstly solved some problems related CPU load due to O.S., and also extended RAM from 8 to 16 gB with no result.
Also set in the preferences to use only the high performance video card (AMD Radeon HD6770M) in order not to use the integrated graphics card to keep the CPU more comfortable.

After some days when I almost went crazy, I found out that the problem is caused by the external screen I have connected to my mac. It is a LG ultrawide screen

When I disconnected the screen from the computer everything came good even with small buffer.

Now the problem is that I need to use back my expensive screen , so what can I do?. The first question would be: If I change the thunderbolt cable that connects my MAC to the LG screen could it works or should I change the graphic card? Maybe any other suggestion.

Thanks in advance.