Mac laptops : MacBook Pro mysteriously shutting down

MacBook Pro mysteriously shutting down

I have a MacBook Pro with 2.2 GHz Intel Core 2 Duo that I bought in 2007. It is currently operating with Leopard. Early in August, I put it to sleep, came back and it wouldn't wake up or even restart if I held the power button. Took it to the Genius Bar to find that the logic board (supposedly) needed replacing. I get it back and within the same week that I get it back, it starts randomly shutting down. It would be plugged in and not overheating when it would shut down. It would also let me hit the power button & just power up again. It got to the point where it would not do it for a week or two and then one day I had issues with when I would power it up first thing in the morning, it would start to boot and would crash in the middle of booting. I would have to try multiple times to get it to start. On one or two occasions, the screen would flicker before crashing when I would be powering up. I took it to the Genius Bar again and left it overnight for diagnostics. They said that they couldn't find any hardware failures, which obviously can't be the case or else my laptop would be working fine. It had tapered off again and then just this week, it has shut down multiple times. Twice, in fact, just today. It's to the point now where in order to get it to power up after it crashes, I have to take out the battery, hold the power button for about 30 seconds, put the battery back in and then it will let me restart. After I powered it up after the 2nd crash today, the screen flickered about a total of 6 times at random intervals. I also just found out last week that the ethernet card is now dead. I'm reluctant to replace it and am considering just selling my MacBook Pro and buying the Mac Mini & a monitor. Any suggestions on how to deal with this would be greatly appreciated.