> Первое что нужно сделать, это проверить нагрузку процессора в моменты выпадения траффика,
> а потом уже все остальное (mtu/mss например). Коробочка слабенькая и ОЧЕНЬ
> старая по нынешним временам.

на нее не слишком большая задача, должно хватать.
вот, о чем должно говорить

FastEthernet0 is up, line protocol is up
  Hardware is PQUICC_FEC, address is 000c.308e.ba90 (bia 000c.308e.ba90)
  MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation 802.1Q Virtual LAN, Vlan ID  1., loopback not set
  Keepalive set (10 sec)
  Full-duplex, 100Mb/s, 100BaseTX/FX
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:00, output 00:00:00, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 3000 bits/sec, 2 packets/sec
  5 minute output rate 3000 bits/sec, 2 packets/sec
     646 packets input, 95099 bytes
     Received 94 broadcasts, 0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog
     0 input packets with dribble condition detected
     569 packets output, 80765 bytes, 0 underruns
     0 output errors, 0 collisions, 5 interface resets
     22 unknown protocol drops
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier
     0 output buffer failures, 0 output buffers swapped out
FastEthernet0.1 is up, line protocol is up
  Hardware is PQUICC_FEC, address is 000c.308e.ba90 (bia 000c.308e.ba90)
  Internet address is x.x.x.x/x
  MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation 802.1Q Virtual LAN, Vlan ID  10.
  ARP type: ARPA, ARP Timeout 04:00:00
  Last clearing of "show interface" counters never
FastEthernet0.2 is up, line protocol is up
  Hardware is PQUICC_FEC, address is 000c.308e.ba90 (bia 000c.308e.ba90)
  Internet address is x.x.x.x/x
  MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation 802.1Q Virtual LAN, Vlan ID  20.
  ARP type: ARPA, ARP Timeout 04:00:00
  Last clearing of "show interface" counters never
FastEthernet0.3 is up, line protocol is up
  Hardware is PQUICC_FEC, address is 000c.308e.ba90 (bia 000c.308e.ba90)
  Internet address is x.x.x.x/x
  MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation 802.1Q Virtual LAN, Vlan ID  30.
  ARP type: ARPA, ARP Timeout 04:00:00
  Last clearing of "show interface" counters never
NVI0 is up, line protocol is up
  Hardware is NVI
  Interface is unnumbered. Using address of NVI0 (0.0.0.0)
  MTU 1514 bytes, BW 10000000 Kbit/sec, DLY 0 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation UNKNOWN, loopback not set
  Last input never, output never, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     0 packets input, 0 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     0 packets output, 0 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 unknown protocol drops
     0 output buffer failures, 0 output buffers swapped out