Networking – Why does the gigabit switch lose connectivity

I have a very simple home network system. I'm trying to replace an older 10/100 4-port switch with a new gigabit 8-port switch to support additional computers without much luck.

Configuration is:

1. Cox internet cable service directly into a NetGear C3700 Wifi Cable modem router (we do not subscribe to cable TV). Modem wifi is turned off for now but might be used later. The router has two gigabit ports; one feeds several computers in the basement via a NetGear GS308 8-port switch (which works fine now), and the other drives a cat6 cable to the upstairs.

2. The upstairs network currently has a Linksys EZXS55W 5-port 10/100 switch which works great but doesn't have enough ports to support a new additional PC. I've tried replacing it with both a D-Link DGS-108 and a NetGear GS308 8-port gigabit switch, but when I do, I lose internet connectivity on the upstairs PCs. Pings to the cable modem time out more than 50% of the time, and browsers on the upstairs machines can't get to the outside.

If I use the 10/100 switch with a single PC behind it upstairs it connects to the modem, receives an IP address via DHCP fine, does not drop any pings to the modem at all, and connects to the internet well. If I simply replace the 10/100 switch with one of the gigabit switches, most of the pings fail and I have extremely limited internet access.

This isn't rocket science, it's simply replacing an old switch with a new switch, which should be bulletproof. But I can't get it to work with any combination of cable plugin order, power cycles, etc. Is there something about 8-port gigabit switches that they're not telling me (yes I've read the manual many times over, it's not that deep)? The only difference I can see is that the 10/100 switch has a dedicated WAN port (which is what I plug the cat6 going to the modem into) and the 8-port has auto-sensing.

Please help…thanks.

UPDATE: I've ruled out cabling and other problems. I took one of the upstairs machines to the basement, plugged it directly into the router's "upstairs" gigabit port with a brand new 10' cat6 cable and verified I could always connect (download speeds > 100Mbps). Did the same thing with a second new 10' cat6 and it also worked fine, > 100Mbps. Now I have two proven cables, so the cables/router/PC are not the problem. Putting the old 10/100 switch between the router and the PC with the known good cables dropped it to ~18Mbps but it was still solid, no dropped pings. Replacing the 10/100 switch with either the D-Link or the NetGear gigabit switches dropped pings to the router more than 50% of the time ("Request timed out") even though the speed indicators on the switches showed they were connected at gigabit speeds. The connection was bad enough that I couldn't even open a browser to perform speed tests.

I took the PC back upstairs and plugged in the "upstairs" cable directly to it (100' cat6 routed through the furnace chase). It worked fine, pulling in ~80Mbps consistently. This was in the evening and Cox sometimes throttles bandwidth even for customers who are paying for the highest speeds (my contract reads 125Mbps), and it was obvious that the connection was being throttled at this time so I'm not too worried about the speed drop.

However, once I use one of the gigabit switches on the end of the 100' cable and try to fan out to the existing machines upstairs it all goes dark again…dropped pings, no connection, just like in the basement. It's bizarre.