Random shutdowns of MacBook Pro

Hello.

I have the following MacBook Pro mid 2011 10.6.8:

MacBookPro 8,2
Intel Core i7
2.2 GHZ

150GB free disc space

I had a liquid damage half a year ago, after which components were replaced (battery, logic board, etc.) in two repair sessions. It worked fine for four months, however now I started having random shut downs (just turning black with no apparent reason). At first they were sporadic, but now they grew regular.

Some more odd facts:

Most of the times, I can only use the macbook when plugged in, and it shows that it is not charging anymore (it says battery has to be replaced). However this only happens sometimes. Sometimes the battery is still working and it doesnt show that it has to be replaced. Sometimes it doesn't even work when plugged in.

I witness the shut downs much more often lately when opening Skype.

Sometimes I was able to induce the shutdowns by moving it around (at least I think that was the reason). Doesnt work always though.

When I connect it to the charger, the MacBook immediately turnes itself on without me pressing the power button. It also restarts automatically after the random shutdowns. It even restarted once when it was closed.

Sometimes it needs maybe 20 attempts to launch with random restarts during the starting process, however, once it's turned on it sometimes works for hours with no shutdown.

I had it checked at the Apple Store when the problems started occurring, however they did not find anything wrong with the hardware in a week-long check. But soon after I got it back the problems started again.

I wondered if this links to a known problem? Could it be a problem of the OS, as they mentionend something like this in the store? Thanks for help!

 

Thanks, I think I might try that. However, when I had it checked in the Apple Store a couple of weeks ago, it did not show anything wrong with the hardware.
also, this problem already occurred after the first liquid damage repair. that's why in the second repair parts were replaced again (including battery and logic board). however, this did not seem to fix the problem in the long run (it worked for 4 months).