If your computer is installed much external accessories such as portable hard disk or web cam and/or your computer is qute updated (not more than 6 months),most of the hardware inside will use up much voltage and so 火牛 maybe the problem. Actually, aound 350-400w 火牛 is enough for home use.
Also, you should check the joints of the newly installed hardware to see whether the joints are loosed or not.