As an update: I by now have my replacement Dell, so I could test my pfSense1 to the memory test. It has been running memtest86 for 24 hours with no problem whatsoever.
I also discovered in the bios the motherboard will automatically scale back the frequency; the bios said 'RAM 1600, actual 1333'.
A wise man who knows many, many things whispered in my ear I should try the PSU (thank you, wise man ;D), so this is what I will do next. And else I will remove pfblocker, since that appeared to keep on crashing on line 262 constantly.
Thank you for your help ;D