HP 6xxx serie artifacts with dual monitor setup

Hello,

I'm working for a company in Holland and we are deploying HP 6200 SFF with a dual monitor setup al followed:

vga to 24inch

dp    to 17inch or 19inch

We deployed 5 pc's with our image and all the users are having artifacts, stripes and bloks. Doesn't mather which driver we are using, the 2011 version or the 2013 version. We also raised the bios version to the latest available...it also seems to occur with one monitor but we are not sure....we asked users to disable one monitor when the problem occurs. 

We already replaced the cabels etc

Any suggestions?

regards,

Roy