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.