diff --git a/pub.sh b/pub.sh
index 917931f..da11441 100755
--- a/pub.sh
+++ b/pub.sh
@@ -4,4 +4,5 @@ set -e
 BASE_URL=${MIRROR_BASE_URL:-"https://mirrors.tuna.tsinghua.edu.cn/dart-pub"}
 UPSTREAM_URL=${TUNASYNC_UPSTREAM_URL:-"https://pub.dartlang.org/api"}
 echo "From $UPSTREAM_URL to $BASE_URL"
-exec /pub-cache/bin/pub_mirror --upstream "$UPSTREAM_URL" --verbose --overwrite --delete --connections 10 --concurrency 10 "$TUNASYNC_WORKING_DIR" "$BASE_URL"
+if [[ $RANDOM -lt 683 ]]; then echo 'Refresh Index Files'; EXTRA=--overwrite; fi
+exec /pub-cache/bin/pub_mirror --upstream "$UPSTREAM_URL" --verbose $EXTRA --delete --connections 10 --concurrency 5 "$TUNASYNC_WORKING_DIR" "$BASE_URL"