XPS 8940, possessed?

I have a 1.5 year old XPS 8940 that is giving me serious trouble. About a week ago it stopped working: the  power light was ON but the screen just went into power saver mode. When I hard reset it from the button, it would show the Dell logo for a couple of seconds then again the screen would go into power saver mode. I could not even go into the boot menu. More strangely, if I shut it down from the button (long press) it would start by itself in a couple of seconds. The only way to keep it shut down was to actually pull the power cable from the back. It was as if the "sleep mode" would survive a hard reset and the computer would "wake" from it by itself. I should mention that the "sleep" mode never really worked on this computer. It would go to sleep for a few seconds and then wake up. Then again go to sleep, and wake up again. 

I tried everything: pulled the two RAM sticks and tried it one by one, tried different RAM,  pulled the GPU and tried a different one, pulled the CMOS battery, replace the CMOS battery. Nothing worked. Finally, I gave up and took it to a professional. One hour after I leave it at his place, the guy sends me a picture with the computer working: "I plugged it in and just works.". There was some complaining with BIOS time (probably due to me pulling out the CMOS battery), but that's it. Whatever, glad it worked, I took it back home, plugged it in, and indeed it worked fine. The time was wrong, so I went into the BIOS and fixed it. 

Two days later (this morning) again the same problem! Exact the same thing as before! So infuriating! The only thing I can think of is that it has, somehow something to do with the sleep mode. Normally, I shut the computer down at night, since, as I said before the sleep mode wasn't working. But last night I forgot to do that. I am guessing repeated sleep/wake cycles mess something up. But how can this survive a hard reset, does this make any sense? I am reluctant to take it again someplace, as it again might work and they can replicate the problem to fix it. 

Any idea what the problem might be and how to permanently fix it? Help would be greatly appreciated it!