Keyboard keys randomly stop working? Please help!

Macbook Air Mid 2011

the third row of my keyboard randomly stops working (qwertyuiop)

The keys would need to be pressed down with a lot of pressure in order for them to start working properly again. However, if for example i press down on the "r" for too long, sometimes the rest of the row would start working fine again. I used to have a virus and when it got removed the keyboard worked perfectly fine again (coincidence or not?) and after a month it started the same issue again.

I downloaded Bitfender Virus Scanner from the AppStore and it did not detect any thing on my laptop.

Is the problem in the software or hardware? Also when the issue came back i quickly logged off of my account and entered as guest and the issue was still there.

Thank you in advance!