Someone who properly understands the signal levels will state if they are in tolerance or not.
Do you get the disconnection issues if you have one PC connected direct to your modem?
Normally if using a router you will present only one IP, the one allocated by the Virgin Media network to the internet. not different ones per PC. Your single external IP is dynamically issued by VM. Therefore how are you allocating an IP per computer
I note there is confusion in this thread between the word router, and switch - exactly what do you have and how have you got everything wired up. Normally you need it setup modem <> router <=> computers