Well, it could be that your uplink is saturated, but if things are set up correctly, then you should be able to prioritize the gaming traffic. Your gaming computer is an ethernet connection and not wireless, right?
Limiting the upload speed of your PTP program will likely help, but you need some type of QoS to prioritize traffic to your gaming computer. It looks like your router is supposed to have that. Some QoS strategies are automatic, and some need to be set up. Some routers have an option to set high priority to one of the ethernet ports on the switch. So if your computer is on port 3, set that to high and the others to low. If you're having problems then obviously the automatic settings aren't working. See if there's a QoS or "gaming" tab where you can specify rules for prioritizing traffic.
Another option is to get something like the Hawking HBB1. I actually have had a lot of luck with that product. It has some type of QoS algorithm in it that prioritizes gaming and VOIP traffic. It really helped me out a lot, mainly on my VOIP but it also improved latency in gaming. You should be able to pick one up for around $30-$40 if you look around.
Hope this helps. Good luck.