I went to newegg.com and used their wattage calculator and put in your specs (approx.) and it returns a recommended PSU of 373 watts... now you have a 400 but 400 is the maximum output.

So it could still be a power problem if your PSU isn't a very good one. If it's low quality it could say 400 watts but really be a terrible performer.

I use a 550 watt PSU and my specs are a bit higher than yours and I don't every have power problems.

Maybe get speedfan or some other temperature reading programs so you can see if you GPU or CPU are overheating.

But first of all, try some different drivers for your graphics card.