Xperia Active :: Changing Screen Brightness Started With A Flickering Screen

My Xperia Active is barely 3 weeks old, running Android version: 2.3.4 and latest build: 4.0.2.A.0.62. My screen brightness is kept at maximum, I have never dropped this phone or abused it and rest of it's functions are running fine. Barely a week in, it seemed to have a faintly flickering screen which I thought might have just been my eyes going. This week, the flickering has been replaced by what I would describe as "breathing" - screen brightness goes from max brightness (100%) to gradual dimness (about 78% brightness) to max brightness again - without me touching anything nor am I running out of power. This happens once in a while throught the day, not constantly. Rather alarming as I am getting very attached to this little phone.

I have been checking the SE forums since the 1st week of ownership, the flickering screen problem has been reported occuring to other SE phone models, including the Mini on which the Active has been based. I have wiped all data from my phone and gave it back to the seller for their tech to figure out. My last option if unrepairable, is to ask the seller to replace the unit. what causes the screen brightness to dim then brighten by itself (despite the latest build and software).