Archaic59 :
It's possible you need to update your graphics drivers. You can get them from Nvidia website. Another possibility is high temperatures from the CPU or GPU. You can use a free prog. called HWinfo to check temps and power supply voltages. I'd do that first and verify newest Nvidia drivers too.
The only thing I see wrong as far as hardware goes is the generic type power supply. I can not guarantee the power supply is causing your problem but a gaming system should have something of better quality. I'm going to assume you are in Netherlands from the PSU maker you have in your build?
If you decide to replace the PSU and need a low cost option for a higher quality unit this model from Antec, made by Delta Electronics, is worth considering-
http://www.pixmania.nl/pc-voeding/antec-vp550f-550-w-pc-voeding/22549428-a.html
It indeed was the PSU, I just bought a new one and I've been playing for 2 hours now without any stalls.
Before I collected my new PSU I did check the core temps. and voltages these where okay.
Then I checked on all drivers of the computer if there was any updates available, many of them I had already installed (didn't do the trick). Now two hours in to Project Cars and still didn't crash on me!
Thank you for the solution.
Best regards, Rik
P.s. I do live in The Netherlands unfortunately