Hi Martin,
ok, I had a look at your system details from HP offical QuickSpecs that is
http://h18000.www1.hp.com/products/quickspecs/13276_na/13276_na.PDFYour system detail is ;
HP Z400 Workstation
Intel Xeon Processor W3680 3.33 GHz, 12MB cache, 1333 memory, 6.4 GT/s QPI, Six-Core, HT, Turbo
475 watts wide-ranging, active Power Factor Correction, 85% Efficient
RAMs , HDDs, .... etc
You have 80Plus power supply and it is 85% Efficient that means its total consumption is 475 watts , however it is using %15 of 475 watts for its self to make 220v AC > 12v and 5v DC. So you have aprox 410 watts available left..
As I've seen your GPU consumed a total of 227W under load in tests.
You CPU takes 130 W , plus RAMs , HDDs....etc.. so you are lacking of power wattage..
I also made sure depending to HP's offical QuickSpecs. on this link please have a look at page 17
http://h18000.www1.hp.com/products/quickspecs/13276_na/13276_na.PDFYour GPU has almost the same power consumption with tesla c2050. And this offical Example Configuration shows you need 460.40 W where as your current power supply can provide 410 watts available power.
This is a big problem on load. It seems you need to upgrade your PSU however your system has custom build PSU and it has different size... it is really unfortunate, I donno how you can solve this..