Zephko

I had the same problem on my DIY PC, maybe you can try full duplex connection with below methods. 1. Replace current RJ-45 cable with a high quality CAT5E ethernet cable. 2. Disconnect all client RJ-45 cables except yours from the router then restart router. 3. Connect RJ-45 cable to modem of internet service vendor directly without additional personal router. Hope these can help you resolve VAC problem.

I think you can download Corsair Diagnostics via below link(official release) and install it again. http://downloads.corsair.com//Files/Vengeance-PC/Corsair_Diagnostic_7.1.6993.139.zip

Have you downloaded correct Intel Performance Maximizer build for i9-9900K? Core i9-9900K Installer.zip: https://downloadmirror.intel.com/28772/eng/Core%20i9-9900K%20Installer.zip