External monitor going black for few secs, and compatibility findings with Macbook 12" Retina

I have been trying to find a perfect big external monitor for my MacBook 12" Retina with digital multiport adapter. In some forums people were questioning us who want big screens on tiny Macbooks. So let's explain that. I create lots of content and need plenty of screen estate, but my use is basically Chrome and Pages, but I also need something small and light to carry around.

There seems to be very little information available of what works and what does not work. So here is my findings, and problems. Suggestions and solutions are welcomed.

My new Dell U3415W (3440 x 1440, 60 Hz) works perfectly, but goes black for few (3-5) seconds every hours or so. I have only have it for a day. Thought it was a cable issue and tried Dell's High Speed HDMI-cable, but still happening. There is also a MHL HDMI port that gives 50 Hz. I haven't tried it extensively yet.

I am basically working with Chrome alone, and it could happen with freshly booted laptop with Chrome alone and only few pages open. Like a minute ago when writing this.

My previous Philips 4065uc (4K, 30 Hz) worked, but there was underscan (black areas around the screen) that couldn't be adjusted. So the image was closer to 38" than 40". Also 30 Hz is too small for work. Pages do not scroll smoothly, mouse does not move smoothly. The image was also a bit soft. I gave it to a colleague. Haven't been completely smooth experience for him either with a top end Win laptop.

I also tried friend's LG 34" ultrawide 34UM95 quickly. Worked with 50 Hz, although did try only one HDMI-port.

My old Dell 24" FullHD worked smoothly with DVI-HDMI-adapter.

Any suggestions or good known combinations. Luckily I have colleagues who to dump non-working monitors.