Asus PA238Q monitor to Macbook connection issues

In a nutshell, I have a brand new Asus PA238Q monitor and am trying to connect it to work on a 2010 white Macbook (version 7,1) running 10.6.8.

I have a mini-dp port on the mac, and I have a cable which goes from mini-dp to HDMI. No converter, just a straight connection. It's not a cheap cable, and has a gold HDMI connection on it.

I have managed to connect the monitor at 1080p resolution (it is a 23" 1920x1080 monitor) but if I set it to higher than 30hz refresh rate on the Mac display options it displays a black screen which flashes up the desktop image once every 5-6 seconds, and also shows various blocks of solid colour and other random stuff. In other words, it's not working properly. This happens with 50hz and 60hz for ref.

30hz is too slow and results in trails from the cursor and juddering motion on movie playbacks etc.

I have troubleshot it to the point where I cannot identify a specific component problem.

The cable works fine at 1080p/60hz on an LG TV with the Macbook, so it's not the cable or the Macbook as such.

I have also connected a PC via HDMI-HDMI at 1080p/60hz to the monitor with perfect results, so it's not the monitor being faulty.

What on earth is the problem?? It's frying my brain!

I have been on the phone to Apple and Asus and neither can seemingly help. Apple suggest I buy a new cable, but I have subsequently tested it on another monitor and it's fine. I am running out of options here!

Is there some compatibility issue with this monitor on the Macbook I am using? The doccumentation states that it is Mac compliant, so I would like to think not.

Anyone using this set-up and can shed any light?!

Thanks in advance for your time 😉