PC randomly turning off/not booting at all (Vista)

My old desktop PC is a Medion MT7 Pentium D 2.80GHz, 2g Ram, running vista.

I think this maybe a PSU problem, but before I go out & buy a replacement I thought I'd check in with the experts.

The PSU is a FSP300-60GTM

It started not to boot when I had my 2 external HD's plugged in, so I made sure to disconnect them prior to powering it up and it worked fine.

However it started acting up again, and eventually I worked out the DVD drive also needed to be unplugged for it to boot this time.

This worked fine for while however now it randomly just turns off after so long (plugging in my usb-powered external HD will trigger it 50% of the time) + will only boot 100% of the time if I disconnect the c drive in addition to EVERTHING else

When I say it doesn't boot, I mean I press the power button, it powers up for 3 seconds, and then turns straight off with clicking sound from the PSU.

No beeps or any warnings.

It seems to me like the PSU is steadly 'losing' its wattage, as I have to keep disconnecting more & more things to get it to boot, and when I plug them in again once it does boot, it's only a matter of time before it turns off again.
 
Yes, I would agree that it is a power supply problem. Ditch this 300 Watt and go for a good quality 500 watt or bigger
 
Thanks for the quick responce, I went to buy a new one but the guy at the store said to find out my motherboard model first as it might not be able to handle the new PSU?

I just checked it & its a Micro Star ms-7255.

How do I know how much wattage it can handle?

Do I have to buy a PSU around the 300w mark just so the motherboard can handle it?
 
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