Its a thame star its used but was tested in the shop, it was just from a local computer repair/sales store.

Do me a favor--go here:
eXtreme Power Supply Calculator v2.5
Using the "Lite/Free" version-- enter all of your hardware (include USB devices)... and see what it says you need for wattage. You should add at least 50 watts to that total-- I usually allow 75 -100.

This will be the best idea of what you need.

it says 250w, so a 400w should be plenty

Okay... so what have you changed lately. The video card-- the machine ran OK before you got it ?

There is the possibility of a bad unit.

One more thing-- when trying to trouble shoot an issue like this --it helps to have all the hardware info... below is my "canned" hardware speech. I give it to everyone with a suspected hardware problem.

I suggest-- when you repost-- go into "My PCHF" under user CP in menu at top under "edit options" --scroll down the list.
List all your hardware in the spaces PLEASE include brands-- at the top-- there is an option to "allow others to view my specs" click on yes. It saves a lot of time between posts if while you check something I can go look up your mobo manufacturer's site and read the manual.
You can also list your OS...all this is a great time-saver for us-- we can view everything quickly to refresh our memories... as we are all working multiple threads at once.This would be automatic from that point on-- so if you have another problem six months from now-- we can instantly know what you have.

for an example-- click on the drop-menu below PCHF Tech in one of my posts