Laptop shut down it own [Closed]

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Hello,

Sir,
Fault No.1. My laptop Sony Vaio model VPCEG 25 EN, 2 years old working as noted below.
While working it may get its own shutdown at any time after the duration of one or two hours.
As instructed by the user manual causes due to CONDENSATION; i remove the battery and separated it for 5 minutes, then inserting it, press to power on; it started to work, while works after some time, same things may happen again.

Fault No.2.
I have to remove the battery after every usage. Suppose if I leave it on lap top, on the next day it does not start to power on. But this case I have to separate it several hours to again to start.
Generally Laptop works in both main and battery independently or combined. On fully charged laptop indicates that it will work about 4 hour's time on battery use.

Just now heat sink, fan etc., cleaned and thermal paste applied.
Fan revolute its normal speed,
After its shutdown checks are made at heat sink other area, result no excess heat
Is there (1) mother board (2) battery or (3)Virus problem
Window 7 ultimate is current OS.and K7 anti virus loaded.
Kindly inform me what I have to do. Most of the technician saying that mother board has to be changed. Is it right? Though laptop was used by me monthly twice ( rare use) I am not interested to change. Advices are expects from you. And I shall pay your charge for this help.

Yours sincerely,
E.Ramakrishnan, cell No. removed..
54, MMDA colony,
Chennai.600106.
India,
E,mail: ***@***