@ nikoly : I changed the battery, but it just reduced the ration of SoD, but problem remained there.
@Klamath : It seems that everybody it now making hit and try methodology... (just kidding, never mid).
someone even saying about ambient screen brightness software, i also did not use that app ever, but problem remained there.
I used the ROM "Hero 7500+ 4.2.1 neHEROvo mod by Dzhyn" for three weeks, and never has screen of death and screen lock problem (seems the issue was resolved and it is not a hardware issue) . but that ROM does not allow some google apps to be updated, and even GPS does not work there, and also some other small couple of bugs. (so i changed ROM again this sunday)
One thing i wanted to share is that , that ROM has a feature, that it disables WIfi automatically when the phone is screen locked for a long time (may be when phone goes to sleep mode). when i open the screen lock, wifi connects then again automatically, and if screen is locked and opened up again within few minutes (say 4 minutes) then wifi is not disabled, it is connected already.
seemed like, the ROM disabled the WIFi when goes into sleep, and hence the problem did not occur. as Someone suggested also to disable wifi settings to connect in sleep mode, I hope will resolve the issue.
Users please check and revert back. as currently i am not facing the issue. and I do not want to face it again as well ... wish me good Luck