Wi-fi signal keeps dropping?

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My wi-fi used to work perfectly fine until last week. That's when I started to get all these problems.
It either completely disconnects itself from my devices or it will be disconnected for a few moments (within seconds or a minute) and then reconnects and then BAM, it gets disconnected again.

I don't know if it's a internet connection issue as I have no problem connecting to the internet using my desktop computer. It only happens when I'm trying to connect my smartphone, PS3 or tablet to the wi-fi. It's a huge pain when I'm playing online games or surfing the web.

Any ideas why this is happening to me? For your information, I have an old modem (5-year-old) so I guess it's probably the main cause of the problem?

Thanks in advance.