Video Post Error with older GPU's on Newer Motherboards

[MaddMutt]

This main topic is about NVidia x2 video cards that refuse to post on x299/z390 motherboards with i9-10940x & i9-9900kf CPU. I have a EVGA x299 Dark & MSI z390 ACE - BUT - these same cards work on my ASUS x99/z270 motherboards with i7 5820k & i7-6700k. The video cards in question are the Geforce 9800 GX2, GTX-295, GTX-590, GTX-690. They will not post (MB error code for GPU) as primary display but will post if they are installed as secondary cards?
I have the - will only post as a secondary GPU - with some of my AMD/ATI HD-5870 (x3), HD-5850 (x2), and HD-5970. <- This is not so bad because the HD-5970 WILL CF with the HD-5870 BUT the NVidia x2's will not SLI with their basic version :-( They also work correctly when installed in an older motherboard?
Has anyone else had/have this problem with older GPU's and newer MB's?

Thank You For Your Time

 

[magellan]

I couldn't get my old Radeon 6970 to boot on my x99 'board until I manually set the PCIe slot to 2.0.