I had the same problem, but the cause might not be your cause.
I play Quake 3 through a firewall to test it. If the PC and firewall software can keep up with Q3, I'm happy with it.
I was losing connections while playing Quake after setting up my first SME Server, however.
Turned out to be the P1 motherboard I was using. It was a TX chipset that only supported ATA33. The Drive was ATA66, and thru a bug in the bios, the mobo would actually try to talk to the drive at ATA66.
This seems to be a common problem with older boards built at a time when ATA33 was as high as any HDD could go.
The system would hiccup when trying to write to the drive and lose connections for just long enough to bring it all down. After a short time, things would start working again.
Anyway, the point is, it may be a hardware problem rather than SME Server.