HDMI external display recognized when off, does not work when turned on

I have just bought a MacBook Pro 13" (early 2015 version) with Yosemite 10.10.2 and I'm having trouble trying to use my 23" external display (LG 23EN43V-B).

I connect my external display using an HDMI cable (I have no Thunderbolt-to-VGA or -DVI adapters) and, when I connect the display turned off it's recognized by Mac OS X as "23EN43". I can set a resolution, a color scheme... everything seems OK.

But as soon as I turn the external display on both screens (MBP's and the external one) start to turn on and off continuously, as if the Mac was trying to initialize the external display without success, then trying again, and again... until I disconnect the external display, or just turn it back off.

And the most interesting thing is, of the about 30 times I tried to connect and disconnect, one of them it worked alright! (and it was not the first time). The image appeared perfectly on the external display, I could use it, move windows to it, etc... until I thought "let's reboot and see if this is fixed for good". And no. When I rebooted, the problem appeared again 😟

I tried the same HDMI cable on my TV (also an LG) and it works perfectly with the MBP 100% of the times, so it seems to be something specifically with my external display. Also, this external display works 100% OK with Windows 7, Linux Mint and Ubuntu.

I already tried resetting the NVRAM. Nothing changed 😟 I also tried deleting the com.apple.windowserver.*.plist files in /Library/Preferences and ~/Library/Preferences/ByHost that some people recommend in other support tickets (and then restoring them back after checking that it didn't work either...)

Any clues please? I'm pretty frustrated...

Thanks so much,

Cluny