Slow after damaging 2 HDDs

So there's a bit of a mystery.

My Macbook started getting really slow, but a different kind of slow. It eventually loaded the system Yosemite, but then apps took a long time to load.

And even after loadning and having gone to the RAM (8GB) it was still slow.

The IT guy said it was the HDD, so we replaced it .... after some time the same thing started happening. Even right after replacing the HDD I never felt it was as fast as it should be, but I thought it was just my impaired geeky judgement.

Then it got real slow. I could not even post things to Facebook, open new tabs, write e-mails without waiting for the already typed letters to show up.

When I woke it up it would show me the apps, but would take ages to let me use them.

The IT guy said all the rest was fine, at least he said he tested everything.

IMPORTANT: a while ago, the computer would turn off all of a sudden depending on how I grabbed it. And then it started showing a message that the battery might have to be serviced. But it disappeared after the guy fixed the battery bad contact thing.

Now I have purchased a new SSD (HyperX Savage) and we're having all sorts of weird problems again. I won't let us install OSX, it acts just like when I was using Pages or the browser, even in that installation interface (which is greatly lighter than the system itself) it freezes a LOT, the cursor freezes, the typing thing shows typed letters only seconds later.

Does anyone have any clue of what might be going on?

A SATA cable? RAM? Motherboard? Battery issues? Processor?

:( I'm afraid it'll damage my SSD just like it did to the new HDD we had installed. The IT guy tested the HDD and it had all sorts of bad sectors and damaging, A 4 month-old NEW SSD.