That's definitely a pretty strange symptom. It may be a sign that the method you're using to connect to the router has problems. Try connecting only via LAN cable, with WiFi switched off, and also try swopping out the LAN cable in case there is an intermittent break in the cable.
You should also be able to see if there is latency to your router from a traceroute, and you can measure any packet loss by pinging the router.
A good place to start would be to check the firewall settings on your PC, and then disable firewall and anti-virus completely. If your AV is set to a strcit setting it could be scanning every outgoing packet and slowing down your traffic.
If you've used different routers, try isolating whatever is common - the PC, cabling or any network devices like switches that you may have continued to use.