Default [Q] HELP Xperia U stuck in bootloop

the last couple of weeks i almost didn't have any cellphone signal and sometimes when i treid to turn on the phone it just wouldn't turn on.
yesterday i had this problem again, but when it finally turned on it got stuck in a bootloop. i am kind of a noob when it comes to flashing and stuf so i deceided flash it using sony pc companion. but i also treid flashtool. i flashed it with stock 4.0.4 ISC but when i turn on the phone now it is really slow and it keeps giving this error that system proces has stoped working. when i pres wait it gives the error again and if i press close it restarts the phone and gets stuck in a bootloop again.
i don't understand this problem and flashing it again doesn't help.
does anybody know if this is a software problem and how i can fix it? or if its a hardware problem in witch case i'm f*cked.

i was running stock sony frimware and it was rooted, just to remove some bloatware. bootloader is still locked.
i hope anybody can help me, thanks