Crash! Acer Aspire 4738G

Hi, I'm having frequent crashes with my laptop.
Model : Acer Aspire 4738G
CPU : Intel Core i3 (1st gen)
GPU : AMD ATI Radeon HD 5470 - 512MB
RAM : 2GB DDR3
HDD : 320GB (Able to use with both AHCI and IDE)

Here's how it all started;

1. During the last 3 months I encountered loud fan noises and lags when playing games. And a lot of heat from the laptop.
2. So, I decided to buy a thermal (later found out that it was some cheap pale-colored Chinese-made one) then I cleaned out the fan, heatsink and removed the dried-out thermal. Added the new thermal (read a lot of online tutorials and videos) After that, at first, there were some frequent crashes but it went OK for about a month.
3. Then it started crashing again. I reapplied thermal like so many times and there was one time, I thought it was some HDD error. So, I used Partition Wizard Home Edition 7.5 bootable disc and formatted the entire drive, rebuilt MBR, created two partitions. (Acer Recovery partition was long lost so I gave up on that one.) Thank goodness the progress went flawlessly. I risked my HDD for it.
4. After that everything went fine like installing a fresh Windows 7 Ultimate, installing drivers, rebooting for drivers and software. (The previous BSOD error of choosing HDD AHCI mode is now fixed so I decided to say this way.) Everything went smoothly when on the next day, it started crashing again.
5. Since the laptop does not crash on BIOS and boot menu, I used this method/trick of leaving the laptop on in BIOS for about 15mins and reboot Windows. Sometimes, it lasted for 6hours, sometimes 12 before crashing. Most of the time it crash. It became worse, even opening Firefox, Slender game makes it crash.
6. I gave up and send it to a local service center, since I lost my warranty a year ago. I think they added a new thermal when I explained how it crashes (hope it's Arctic Silver). After about 3 days, they said it's BIOS error and motherboard error and can't fix it.
7. When I came back from taking it back, I searched for BIOS recovery and found ways. Since I had an error when updating BIOS from Windows (not DOS), to some older version to v1.16, I tried to downgrade ( I could say) to v1.14. I used the Fn+Esc and ZQ8x64.fd in a formatted USB method. It went fine.
8. Then I could go to Windows 7 Recovery mode, and went into command to check disk, both partitions went fine with no errors. Then I decided to do a Memory Diagnosis. It rebooted and when it came to diagnosing, at the start, it crashed again and again.
9. Took out the battery, used only the main line, no use. Still crashing. Battery only, no use.
10. Used default setting in BIOS, no use.
11. In BIOS, Quiet Boot, Network Boot, F12 Boot Menu, D2D Recovery, IDE Mode; I enabled/disabled in many ways, but still no use.
12. Since I had another Acer laptop, I exchanged the charger adapter, no use. Thought the USB ports was hogging all the powers and make it crash so I unplugged all the ports when booting, still no use. I don't thinks so.

Service said it was motherboard error and said it was hard to find (since I live in Myanmar) and would cost about $150. My mind couldn't accept it and still want to repair my old-trusty sidekicking pal.
Sorry for the long post, I hope it has enough details to figure out what the problem is. Thanks in advance.