So I have a Raspberry Pi B+ - I've had it for several years now, which I think may be the cause of this problem, though I'm not sure how.
Basically, it's fine when I have just a keyboard plugged in, albeit a bit slow, and likewise when I have a keyboard and mouse plugged in, but when I plug in my USB network adapter a little lightning bolt symbol flashes at the top right of the monitor, and power to the keyboard is repeatedly cut off, then turns back on, then gets cut off again, and so on.
When this is happening, the red LED on the PI itself flashing in time with the power to the keyboard (that is, when the keyboard turns off, the light flashes.)
I'm pretty sure it's not the network adapter's fault, since it used to work fine.
If it's relevant, my Pi was overclocked for about a week prior to this, though I didn't use it much in that period. It was overclocked to the turbo setting, and I think this may have been the issue. I've un-overclocked it now, but the issue persists.
So, is it time to get a new Pi? Or is there something else I can do?