I have the same problem.

kern.log excerpt:
Jun 23 07:51:06 myhost kernel: [199555.063000] NETDEV WATCHDOG: eth1: transmit timed out
Jun 23 07:51:06 myhost kernel: [199555.063000] eth1: transmit timed out, tx_status 00 status e601.
Jun 23 07:51:06 myhost kernel: [199555.063000]   diagnostics: net 0cc8 media 8880 dma 0000003a fifo 0000
Jun 23 07:51:06 myhost kernel: [199555.063000] eth1: Interrupt posted but not delivered -- IRQ blocked by another device?
Jun 23 07:51:06 myhost kernel: [199555.063000]   Flags; bus-master 1, dirty 826375(7) current 826375(7)
Jun 23 07:51:06 myhost kernel: [199555.063000]   Transmit list 00000000 vs. dfd2c660.
Jun 23 07:51:06 myhost kernel: [199555.063000]   0: @dfd2c200  length 8000002a status 0001002a
Jun 23 07:51:06 myhost kernel: [199555.063000]   1: @dfd2c2a0  length 8000002a status 0001002a
Jun 23 07:51:06 myhost kernel: [199555.063000]   2: @dfd2c340  length 8000002a status 0001002a
Jun 23 07:51:06 myhost kernel: [199555.063000]   3: @dfd2c3e0  length 8000002a status 0001002a
Jun 23 07:51:06 myhost kernel: [199555.063000]   4: @dfd2c480  length 8000002a status 0001002a
Jun 23 07:51:06 myhost kernel: [199555.063000]   5: @dfd2c520  length 8000002a status 8001002a
Jun 23 07:51:06 myhost kernel: [199555.063000]   6: @dfd2c5c0  length 8000002a status 8001002a
Jun 23 07:51:06 myhost kernel: [199555.063000]   7: @dfd2c660  length 8000002a status 0001002a
Jun 23 07:51:06 myhost kernel: [199555.063000]   8: @dfd2c700  length 8000002a status 0001002a
Jun 23 07:51:06 myhost kernel: [199555.063000]   9: @dfd2c7a0  length 8000002a status 0001002a
Jun 23 07:51:06 myhost kernel: [199555.063000]   10: @dfd2c840  length 8000002a status 0001002a
Jun 23 07:51:06 myhost kernel: [199555.063000]   11: @dfd2c8e0  length 8000002a status 0001002a
Jun 23 07:51:06 myhost kernel: [199555.063000]   12: @dfd2c980  length 8000002a status 0001002a
Jun 23 07:51:06 myhost kernel: [199555.063000]   13: @dfd2ca20  length 8000002e status 0001002e
Jun 23 07:51:06 myhost kernel: [199555.063000]   14: @dfd2cac0  length 8000002a status 0001002a
Jun 23 07:51:06 myhost kernel: [199555.063000]   15: @dfd2cb60  length 8000002a status 0001002a
Jun 23 07:51:06 myhost kernel: [199555.063000] eth1: Resetting the Tx ring pointer.

uname -a:
Linux scatology 2.6.20-16-lowlatency #2 SMP PREEMPT Thu Jun 7 20:23:03 UTC 2007 i686 GNU/Linux

lspci:
00:00.0 Host bridge: nVidia Corporation nForce3 250Gb Host Bridge (rev a1)
00:01.0 ISA bridge: nVidia Corporation nForce3 250Gb LPC Bridge (rev a2)
00:01.1 SMBus: nVidia Corporation nForce 250Gb PCI System Management (rev a1)
00:02.0 USB Controller: nVidia Corporation CK8S USB Controller (rev a1)
00:02.1 USB Controller: nVidia Corporation CK8S USB Controller (rev a1)
00:02.2 USB Controller: nVidia Corporation nForce3 EHCI USB 2.0 Controller (rev a2)
00:05.0 Bridge: nVidia Corporation CK8S Ethernet Controller (rev a2)
00:06.0 Multimedia audio controller: nVidia Corporation nForce3 250Gb AC'97 Audio Controller (rev a1)
00:08.0 IDE interface: nVidia Corporation CK8S Parallel ATA Controller (v2.5) (rev a2)
00:09.0 IDE interface: nVidia Corporation CK8S Serial ATA Controller (v2.5) (rev a2)
00:0a.0 IDE interface: nVidia Corporation CK8S Serial ATA Controller (v2.5) (rev a2)
00:0b.0 PCI bridge: nVidia Corporation nForce3 250Gb AGP Host to PCI Bridge (rev a2)
00:0e.0 PCI bridge: nVidia Corporation nForce3 250Gb PCI-to-PCI Bridge (rev a2)
00:18.0 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] HyperTransport Technology Configuration
00:18.1 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] Address Map
00:18.2 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] DRAM Controller
00:18.3 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] Miscellaneous Control
01:00.0 VGA compatible controller: nVidia Corporation NV44A [GeForce 6200] (rev a1)
02:09.0 Ethernet controller: 3Com Corporation 3c900B-TPO Etherlink XL [Cyclone] (rev 04)
02:0d.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL-8169 Gigabit Ethernet (rev 10)

/proc/interrupts:
           CPU0       
  0:    2932311   IO-APIC-edge      timer
  1:       4948   IO-APIC-edge      i8042
  7:          0   IO-APIC-edge      parport0
  8:          3   IO-APIC-edge      rtc
  9:          0   IO-APIC-fasteoi   acpi
 14:      35882   IO-APIC-edge      ide0
 15:      26457   IO-APIC-edge      ide1
 16:      29201   IO-APIC-fasteoi   ohci_hcd:usb1, libata
 17:         17   IO-APIC-fasteoi   ohci_hcd:usb2, libata
 18:     129633   IO-APIC-fasteoi   ehci_hcd:usb3, NVidia CK8S
 19:     292128   IO-APIC-fasteoi   eth0
 20:     290715   IO-APIC-fasteoi   eth1
 21:     347604   IO-APIC-fasteoi   eth2, nvidia
NMI:          0 
LOC:    2931995 
ERR:          0
MIS:          0

lspci -vvv excerpt:
02:09.0 Ethernet controller: 3Com Corporation 3c900B-TPO Etherlink XL [Cyclone] (rev 04)
        Subsystem: 3Com Corporation 3C900B-TPO Etherlink XL TPO 10Mb
        Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
        Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
        Latency: 64 (2500ns min, 12000ns max), Cache Line Size: 64 bytes
        Interrupt: pin A routed to IRQ 20
        Region 0: I/O ports at 8c00 [size=128]
        Region 1: Memory at fdfff000 (32-bit, non-prefetchable) [size=128]
        [virtual] Expansion ROM at fde00000 [disabled] [size=128K]
        Capabilities: [dc] Power Management version 1
                Flags: PMEClk- DSI- D1+ D2+ AuxCurrent=0mA PME(D0-,D1+,D2+,D3hot+,D3cold-)
                Status: D0 PME-Enable- DSel=0 DScale=0 PME-