Okay guys... help?
The machine:
Sun Ultra 10, no add-in cards
Built-in 10/100 ethernet (more on that later)
512MB RAM (in case anyone thought it might have 32MB)
The network:
GOOD CABLES! (trust me on this one)
Actiontec GT701-WG, AKA. "A really crappy DSL modem". DHCP server running. Firewall and all the fancy crap is off, a few non-standard ports are forwarded to my main system.
3Com SuperStack II Dual Speed Hub 500
Linksys WRT54G between the DSL modem and the 3Com hub because the modem doesn't like the hub very much. All the fancy stuff has been turned off and the Linksys is acting only as a ethernet hub/wireless bridge.
Five other computers running either WinXP, OSX, or Ubuntu 9.04 and using DHCP. No problems of any kind with these.
The horror:
I can ping most sites but some don't return (microsoft, ebay, etc.) but everything resolves correctly. I'm working under the assumption that the sites which don't return pings are doing this on purpose. Most of the time sites which I CAN ping from the command line give me a "connection refused" or "timed out" error from a browser. Some sites work intermittently and others don't work at all; sometimes I get nothing but Debian's site

. I'm by no means an expert in the world of Linux but I have general troubleshooting skills and enough sense to read. Here's what I've done, from the beginning:
Ubuntu 9.04 - Can't find the CD drive it's loading itself from
Gentoo ?.? (the newest one) - Same as Ubuntu
Debian 5.0.1 - Someone broke the kernel and it won't start X on the Ultra 10. I haven't tried 5.0.2 because I'm getting tired of burning CDs and running out of faith.
Debian 4.0R8 - Installed! But "connection refused" problem described above
FreeBSD 7.2? (the newest one) - Good download & good burn. Couldn't find some install files.
Aurora 2.0 - Installed. Runs fine. Same "connection refused" problem. This is the distro I'm currently running.
Installed the following network cards simultaneously: 3Com 905B-TXNM, Linksys LNE100TX, Netgear FA311 (rev. A2), and some unknown card with Intel S82557 chip. All but the Netgear were detected but the Intel card couldn't grab an IP address from the DHCP server. The 3Com and Linksys cards recieved addresses just fine but had the same "connection refused" problem. I've tried ethernet cables plugged into both the 3Com hub and the Linksys hub; same problem. Therefore, the issue is almost certainly not hardware.
Pulled all of the network cards I'd added and configured IP manually with an address within my subnet but outside the DHCP server's assignment range (guaranteed to be an unused IP). Same problem as before. IP is still configured manually.
Ran a few more searches. The only useful idea I found dealt with setting "echo 0 > /proc/sys/net/ipv4/tcp_ecn" in sysctl.conf (something to do with ECN). Sorry I can't post links, but the poster says it's not a bug. To my way of thinking software which doesn't work with the majority of available hardware is buggy. I tried it but same problem as always.
Current ifconfig result after managing to load google:
eth0
Link encap:Ethernet HWaddr [I don't want to type all that]
inet addr:192.168.0.231 Bcast:192.168.0.255 Mask:255.255.255.0
inet6 addr: fe80::a00::20ff::fec1::7717/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:493 errors:0 dropped:0 overruns:0 frame:0
TX packets: 402 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:287459 TX bytes:46234
Interrupt:224 Base address:0xf000
lo
Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:1686 errors:0 dropped:0 overruns:0 frame:0
TX packets:1686 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:1828055 TX bytes:1828055
Results of "route -n":
Kernel IP routing table
Destination____Gateway_____Genmask______Flags__Met ric__Ref__Use__Iface
192.168.0.0____0.0.0.0______255.255.255.0__U_____0 ______0___0____eth0
169.254.0.0____0.0.0.0______255.255.0.0____U_____0 ______0___0____eth0
0.0.0.0________192.168.0.1__0.0.0.0________UG____0 ______0___0____eth0
As I said, I don't know a bunch about Linux but I'm not prone to falling on the ground and squealing for help until I think I've run out of other places to look for solutions. I've reached that point and am therefore calling upon the knowledge of the 'net. If anyone needs more information I'm quite sure I can oblige and if anyone has suggestions other than slamming my fingers in a door or banging my head against a wall I'd love to hear 'em. Aww heck, if anyone actually DOES think finger-slamming or head-banging might work I'll listen to that too.
-Chris