Xperia Ray :: Display Won't Turn On After Stand-by

Sometimes (actually quite often) when I try to wake up my phone from standby, the display will just stay black with the LED turning white.The screen itself reacts, I can feel it vibrating when I touch it.Same when I recieve a call that would wake the phone or an alarm etc.Taking out the battery fixes the problem temporarily, so does waiting around 10 minutes then try again, but not all of the time.I already tried the following things:

1. I reseted the phone to factory settings. The problem still occurs even if it's just like out of the box.

2. I connected the phone to the computer to check if there were any new updates since 4.0.2.a.0.69 but there weren't any (please correct me if I'm wrong).

3. I took my phone to my carrier (Germany) and they sent it to a service center. I got it back and they said they fixed it (the carrier couldn't tell me what exactly they did though) and it worked fine for exactly one day. So this forum is my last chance, I saw a similar thread here so I think I'm not the only one with this problem. None of the posts could help me though and I was afraid if I posted there my problem would go unnoticed. Please don't be mad at me if that is considered rude or something

I don't want to take the phone to the carrier yet again seeing as I took them almost a month to send me my phone back without even fixing it correctly. I will if I have no other choice as I still have guaranty on this not even half a year old phone, but as long as I can avoid it, I'll try to fix it myself..I have the feeling the problem came with the last update. I don't want to upgrade to ICS as I don't like the look so I'm wondering, will there still be other updates for GB coming out?I don't want to flash the phone back to an older firmware as that would mean unbranding it and makes me lose my guaranty?