Joystick has spiky (variable) output -- get powered hub

If you find that that output of your joystick is jittery, variable, or spiky, instead of plugging your joystick into one of the USB ports on your computer, get a powered USB hub, and plug into that.

Make sure your USB hub is receiving its external power.  Some of them have an LED that lights up when the external power is applied.  I had a problem once where my joystick was plugged into a powered hub, was working fine, then started to give variable output.  It turned out that the power cord to my USB hub had slid out a bit, and the hub was no longer running on external power.  Once I pushed the power plug back in, things started to work well again.

If your joystick was working when plugged into your motherboard's usb port then started becoming variable in output, still try a powered hub.  I've had this happen to me when the motherboard's usb port's power supply starts to give out after a while of use.