These issues usualy fall into two camps.
The first is RPi psu is not up to snuff for whatever reason be that under rated or incorretly labled or just plain flaky manufacturing.
The second is periferals attached to the USB drawing too much from the USB here we have scenario of the punter using a flashy back lit keyboard when the best option is the cheapest USB wired one you can find.
If you don't fall into the second camp suspect your power supply
If you change to a cheap wired keyboard (leave off the mouse) and it still happens I refer you to the previous line.
There is lots of other possibilities but nearly every case reprted here has come down to one of these two.
The ultimate low power is to run headless (see noob bit on wiki) ie. just ethernet cable and power cable and login remote with ssh
Noob is not derogatory the noob is just the lower end of the noob--geek spectrum being a noob is just your first step towards being an uber-geek
If you find a solution please post it in the wiki the forum dies too quick