Macmini can't wake up and I Get a Black Screen

Hi all, this is my first post in this community. Please do help me.

I purchased a Mac mini (2012 late model, OS was originally Mavericks 10.9.1 when the machine arrives at my home. ) in January 2014. I used a “miniDP to DP adapter” to connect the Macmini with my Asus PB278Q Display (the original DP cable that comes with PB278Q is connected to the adapter. The adapter is plugged into the Thunderbolt socket on the back of Macmini). It supports the maximum resolution on my display (2560x1440), but if I use a HDMI cable instead, it only supports up to 1920x1080P. Everything was fine until last week when I attempted to wake up macmini after it had slept for 8 hours or so (I used it in the morning, put it to sleep mode before going to work, and then came back at 5:40 pm hoping to use it again). The Macmini light was on, and I could hear noise. But the screen is black with its power light yellow. When I kept hitting the keyboard, I could hear a “Ding” from Macmini each time, and the PB278Q power light sometimes turned blue, but soon a prompt "No Signal” appeared on the screen, followed by a totally black screen with power led turning back to yellow.

I tried to unplug the adapter from the thunderbolt port, plugging it back in, connecting a HDMI cable with macmini (PB278Q supports both DP and HDMI), nothing helped. Pressing and holding the power button for 6 secs, and pressing it again to let Macmini reboot surely helps each time. I have not done a thorough experiment to tell within how many hours the wakeup has no issues. But I am sure that there is no issues at least within one hour. I.e. if I walk away from my macmini, and it sleeps (set to 10 mins sleep), come backs after not a long while, and hit the keyboard or the trackpad. The screen comes back to life immediately, and I see the “login” dialog.

I called Mac techsupport for three times and talked with their higher level engineers for a long time. They suggested me to do “Command+Option+R+P”. Now I know that is “Resetting NVRAM / PRAM”. But the problem stays.

Does Re-installing help? I just reinstalled last night and won’t be able to judge until tomorrow. At this point, I have the following suspicions, which I hope someone can help:

1. 1. The miniDP to DP adapter might have caused this problem. Replacing one adapter or using a good miniDP-DP cable might be the solution. But I can’t find such a cable or adapter on Apple’s online store. Note it should be Mini-DP to DP or thunderbolt to DP cable. Please suggest where I can get one
2. 2. I doubt that my suspicion 1 is valid, as in my question – why is everything ok when the macmini does not sleep or sleeps a short time? When I work on the macmini, no problem happens. The display works fine, and the resolution is the maximum that the display is capable of. Therefore, why should my suspicion one hold any water?
3. 3. Since the display power led turns blue when macmini detects keyboard hits, does that prove that macmini is innocent, as it successfully sends out a signal to my display, but somehow my display persistently says “no signal”. In this light, can we shrink the problem identification scope to the display, the DP cable and the adapter?

Thank you very much for any help or hint/tips I can get from any of you.

God bless.

Bluebird,

Chengdu, China