Edit: To add, my pings to modem are 3.5ms solid, 8.5ms to the modem gateway and 25ms to Google's DNS servers.
Nothing wrong with that at all. Id be a little concerned with the 3.5ms to your modem if I was a gamer... Rule out the router by going direct and testing if ping times matter. Your modem just may be slower to respond to pings. But the 8.5ms to your ISP is something to be jealous of. This would mean that if you truly have 3.5ms of latency to your modem then your gateway is only 5ms away...

If you have intermittent problems, watch it throughout the day.

When you went from your ata at the modem to the DD-WRT at the modem, you would have been assigned a new public IP address. Probably on a new gateway at your ISP. The other gateway could of had issues.

When my ISP was in its infancy, they had everything running from Windows boxes at the headend. The load balancing sucked and a small number of us were getting pretty good at using cloned MAC addresses from other computers on our own networks into our routers to get our gateway ip to change. We would actually get on DSLR and share the gateway info we were having success with. Not to say for sure this is your issue but you never know...


Pinging 192.168.100.1 with 32 bytes of data:

Reply from 192.168.100.1: bytes=32 time=1ms TTL=63
Reply from 192.168.100.1: bytes=32 time=1ms TTL=63
Reply from 192.168.100.1: bytes=32 time=1ms TTL=63
Reply from 192.168.100.1: bytes=32 time=1ms TTL=63