sounds like you have something installed that causes this, if not then have you transferred any media to it that may contain a virus although its linux rather than windows or mac a virus targeted at them can cause random problems for you so either scan all your media that you transferred and all the emails or remove all downloads that aren't from the stock catalogs.
if it is down to a virus you got then you'll have to re flash, make sure you use upto date os and emmc