Keyboard issues.

[black hawk]

I have a Zebronics ZEB-KO98B PS/2 type keyboard. It's been running for around 4 years. Since the last month or so, I noticed that a particular set of keys that include Escape, 1, a, q, z, tab, and a few more just wouldn't work. For a few times, I just unscrewed the whole keyboard, wiped off the dust and reassembled it. It worked a few times, however only for a few days or hours, but now, even that doesn't do. Is there anything that I can do, or is it time to buy a new keyboard?