Problems with my iMac - HELP

Hello everybody,

My name is Thiago and I have a mid 2011 iMac 27", i5 3.1GHz and 12GB of memory.

In the final of 2012, my mac starts to have a problem. The screen starts to turn off when I was using, I don't know why.

I spend 1 year running to the apple store (iTown here in Brasil) to change the hard disk, the graphic card and after all of it, was the display (as I presume all the time) and they said it will coast above R$ 2.200(aprox. US$ 880).

Then I saw this: https://discussions.apple.com/thread/4041042?start=2&tstart=0

So, I ask to a friend who knows soldering to do the service, but... he can't, and we mess everything. The contact, where we need to soldering, melted!

And, now I use a external screen (my tv) as monitor, but the colors are terrible. And, after this, I don't know why. Everytime I start my computer, the fan speed rises a LOT in seconds and my memory uses too. After my memory goes to 3,5 gb of use, now, 5 minutes of use, goes too 6 gb of use.

I need some help. Why my fan is so fast? Why my memory use rise so fast?

And if you know some great external screen to use on a mac, say it.

Thanks a lot!