To follow up on the subject, I (or better said my computer facturer) fixed the problem.
So a big thanks goes to
www.brentford.com for their great support!
They made the following steps:
- all drivers and software was up to date
- checked all components again with a stress test with various benchmark softwares. they all worked fine except for maxwell render, where it was not possible to work while rendering due to random lag spikes.
- changed all RAM modules to a different kind (as being told by nextlimit to be the main source for problems like this) -> no changement
- changed the grafics card and the mainboard (so basically a new computer

) -> works fine now!!!
they switched a geforce gtx680 for a ati fire pro v7900 and replaced the supermicro x9dai (a pretty new mainboard) with an asus z9pe-d8 ws.
my assumption is, that the mainboard driver in combination with maxwell render caused the problem. I would have expected a bit more effort from nextlimit, since the only software having problems was maxwell render.
if other users have a similar problem with that mainboard, I hope nextlimit or a driver update will be able to fix this.
cheers,
kami