I picked this shell script up years ago that performs an identical function but uses blocks to help with speed. I've never had an issue with the scpresume (I keep it aliased in my 30k .bash_profile), but did have a few issues when transfering very large files (>100MB) over slower links (something not inside your own network). Header is retained with the original author.

throw this into scp-resume.sh or whatever works for you

#!/bin/sh
#
# Speed improvements by using blocks by nitro.tm@gmail.com
#
# This script assumes that you have access to the 'dd' utility
# on both the local and remote host.

# dd transfer blocksize (8192 by default)
blocksize=8192

usage()
{
  echo
  echo "Usage: `basename $0` -u(pload)   \$localfile  \$remotefile [\$sshargs]"
  echo "       `basename $0` -d(ownload) \$remotefile \$localfile  [\$sshargs]"
  echo
  echo "  \$remotefile should be in the scp format, i.e.: [user@]host:filename"
  echo "  \$sshargs are option further ssh options such as a port specification"
  echo "     (-p 1234) or use of compression (-C)"
  echo
  echo "  -u:"
  echo "     \$remotefile may be [user@]host: for uploading to your remote home directory"
  echo "  -d:"
  echo "     \$localfile may be a period (.) when downloading a remote file to the"
  echo "       current working directory."
  echo
  exit 1
}

[ -z "$1" -o -z "$2" -o -z "$3" ] && usage

option=$1
case $option in
  -[uU]*)
    localfile=$2
    remote=$3
    shift 3
    sshargs="$*"

    userhost=${remote%:*}
    remotefile=${remote#*:}

    if [ ! -f "$localfile" ]; then
      echo "!! File not found: $localfile"
      usage
    fi
    if [ x"$userhost" = x"$remote" ]; then usage; fi
    if [ x"$remotefile" = x"$remote" -o -z "$remotefile" ]; then remotefile=`basename "$localfile"`; fi

    echo "==>> Getting size of remote file:"
    localsize=`ls -l "${localfile}" | awk '{ print $5 }'`
    remotesize=`ssh $sshargs "$userhost" "[ -f \"${remotefile}\" ] && ls -l \"${remotefile}\"" | awk '{ print $5 }' `

    [ -z "$remotesize" ] && remotesize=0
    echo "=> Remote filesize: $remotesize bytes"
    
    if [ $localsize -eq $remotesize ]; then
      echo "=> Local size equals remote size, nothing to transfer."
      exit 0;
    fi
    
    remainder=$((remotesize % blocksize))
    restartpoint=$((remotesize - remainder))
    blockstransferred=$((remotesize / blocksize))
    
    echo "=> Resuming upload of '$localfile'"
    echo "  at byte: $restartpoint ($blockstransferred blocks x $blocksize bytes/block),"
    echo "  will overwrite the trailing $remainder bytes."

    dd bs=$blocksize skip=$blockstransferred "if=${localfile}" | \
      ssh $sshargs "$userhost" "dd bs=$blocksize seek=$blockstransferred of=\"$remotefile\""

    echo "done."
    ;;
  -[dD]*)
    localfile=$3
    remote=$2
    shift 3
    sshargs="$*"

    userhost=${remote%:*}
    remotefile=${remote#*:}

    if [ x"$localfile" = x"." ]; then localfile=`basename "$remotefile"`; fi
    if [ ! -f "$localfile" ]; then
      localsize=0;
    else
      localsize=`ls -l "${localfile}" | awk '{ print $5 }'`
    fi
    [ x"$remotefile" = x"$remote" ] && usage
    [ -z "$localsize" ] && localsize=0

    remainder=$((localsize % blocksize))
    restartpoint=$((localsize - remainder))
    blockstransferred=$((localsize / blocksize))
    
    echo "=> Resuming download of '$localfile'"
    echo "  at byte: $restartpoint ($blockstransferred blocks x $blocksize bytes/block)"
    echo "  filesize: $localsize; will overwrite the trailing $remainder bytes."
    ssh $sshargs "$userhost" "dd bs=$blocksize skip=$blockstransferred \"if=${remotefile}\"" |
      dd bs=$blocksize seek=$blockstransferred "of=$localfile"

    ;;
  *)
    usage
    ;;
esac