Strange internet speed issue on my build

[n62]

I am experiencing an odd internet speed issue on my gaming PC that I built last year. I'm paying for 50 mbps from my ISP. On a 4-year-old Dell Inspiron, my download Speedtest results this morning have been as promised: 50.52, 50.01, and 50.01 on tests run within a 10-minute period. However, on my gaming PC, Speedtest results from the same time period (but not measured concurrently with the Dell) have been 15.71, 15.71, and 15.73. Both PCs are on wired connections to the same modem. So, something seems to be bottlenecking the speed on my gaming rig despite that it is way more powerful than the Dell. The specs for both PC's:

Gaming PC (that is getting the slow speeds):
i5 4670k @ 3.40 Ghz
MSI Z87-G45 Gaming
16 mb RAM (Corsair Vengeance)
EVGA GTX 770 SC
Windows 8.1 Pro

Dell Inspiron (that is getting the internet speed as promised)
i5 750 @ 2.67 Ghz
generic MOBO
8 mb RAM
GeForce GTX 560 Ti
Windows 7 Home Premium

I tried the tutorial I found on Tom's for flushing the DNS cache in Windows 8 and deleting all the temp internet files. The problem persists. Anyone have an idea why my gaming rig is so much slower?

Edit: When I run Speedtest on the gaming PC, the needle immediately goes up to the 50 mbps range for about a half second, then drops down and hovers in the 15-16 range.

Thanks

 

[Adam Schneider]

Are you running a powerline adapter? Wi-Fi? Wired?

 

[n62]

Both PCs are on wired connections to a Hitron CGN (I guess this is a router)?
Not sure what a powerline adapter is?

 

[Adam Schneider]

have you tried switching ports at the modem, Such as plug the gaming PC where the other one is and vice Verca to see if it is the ports or the cables that are causing the slow down?

 

[n62]

Thanks - just tried that, and unfortunately the issue persists. Any other thoughts? Could it be a hardware issue?