first time here guys ,so forgive me if i do something wrong. i've just installed a new power supply. the fan stopped working on the power supply that came with the machine. anyway, the new power supply worked good the first night. the next day the pc would not come on. i removed the cover on the cpu tower, while doing this , the machine started up. in about 2 minutes it cut back off. i found that when i tap on the top of the power supply , it makes it work. hope it stays on long enough for me to email this to you guys. thanks for any help

Sounds like either it's a loose connection or a faulty power supply. Check all the connections especially the connections to the swicth and the motherboard. If that doesn't help the I would suggest at looking into the warranty on the new supply and getting it replaced. If it is an ATX type power supply then there is no practical way of testing it so replacement is about the only option.

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mjc