I have had some similar slowdowns, and it seems to have something to
do with the sftp transfers opening up to 8-packet windows.  But it
doesn't seem like any packets are being dropped on the wire.  I have
not turned on debugging to see the counts of retransmits.

Hint: it would be really cool if venus/vice would maintain
  rpc packets transmitted
  rpc packets ack failures
  sftp packets transmitted
  sftp packet ack failures
  disconnections
on a per-peer basis and dump these periodically to the log.
I miss not having 'netstat -p rpc2 -s'.

I did find this in the server log (up 14d 20h).  There seem to be lots of RPC
invalid packets received, and I'm not sure what to make of SFTP's 105
retires, 1837 timeouts and 13123 bogus.

00:14:28 RPC Total bytes:     sent = 31216540, received = 41375275
00:14:28        bytes sent: rpc = 7030315, multirpc = 0, sftp = 24186225, sftp multicasted = 0
00:14:28        bytes received: rpc = 41375275, multirpc = 0, sftp = 0, sftp multicasted = 0
00:14:28 RPC Total packets:   sent = 89840, received = 85902
00:14:28        packets sent: rpc = 57979, multirpc = 0, sftp = 31861, sftp multicasted = 0
00:14:28        packets received: rpc = 85902, multirpc = 0, sftp = 0, sftp multicasted = 0
00:14:28 RPC Packets retried = 520, Invalid packets received = 30333, Busies sent = 705
00:14:28 RPC Requests 29893, Good Requests 28687, Replies 26857, Busies 25
00:14:28 RPC Counters: CCount 41; Unbinds 468; FConns 468; AConns 509; GCConns 0
00:14:28 RPC Creation counts: Conn 63; SL 20; PB Small 13, Med 32, Large 16; SS 4
00:14:28 RPC2 In Use: Conn 41; SS 4
00:14:28 RPC2 PB: InUse 1, Hold 0, Freeze 0, SFree 12, MFree 32, LFree 16
00:14:28 RPC2 HW:  Freeze 0, Hold 0
00:14:28 SFTP:  datas 21980, datar 12241, acks 9705, ackr 3573, retries 105, duplicates 0
00:14:28 SFTP:  timeouts 1837, windowfulls 30, bogus 13123, didpiggy 856
00:14:28 Total CB entries= 30405, blocks = 35; and total file entries = 13042, blocks = 60
00:14:28 There are currently 29 connections in use
00:14:28 There are 5 workstations and 1 are active (req in < 15 mins)
00:14:28 Ether Total bytes: sent = 0, received = 0
00:14:28 Ether Packets:     sent = 0, received = 0, errors = 0
00:14:28 Printing RVM statistics

        Greg Troxel <gdt_at_ir.bbn.com>