diff --git a/piker/fsp/_engine.py b/piker/fsp/_engine.py
index 21688b8b..7bc4510c 100644
--- a/piker/fsp/_engine.py
+++ b/piker/fsp/_engine.py
@@ -254,8 +254,15 @@ async def cascade(
                     # respawn the compute task if the source
                     # array has been updated such that we compute
                     # new history from the (prepended) source.
+                    diff = src.index - dst.index
                     new_len = len(src.array)
-                    if new_len > last_len + 1:
+
+                    # XXX: ok no idea why this works but "drunk fix"
+                    # says it don't matter.
+                    if (
+                        new_len > last_len + 1 or
+                        abs(diff) > 1
+                    ):
                         log.warning(f're-syncing fsp {func_name} to source')
                         cs.cancel()
                         cs, index = await n.start(fsp_target)
@@ -266,24 +273,12 @@ async def cascade(
                     # read out last shm row, copy and write new row
                     array = dst.array
 
-                    # TODO: some signals, like vlm should be reset to
-                    # zero every step.
-                    last = array[-1:].copy()
+                    # some metrics, like vlm should be reset
+                    # to zero every step.
                     if zero_on_step:
                         last = zeroed
+                    else:
+                        last = array[-1:].copy()
 
                     dst.push(last)
                     last_len = new_len
-
-                    # compare again with source and make sure
-                    # histories are index aligned.
-                    diff = src.index - dst.index
-                    if diff:
-                        if abs(diff) < 10:
-                            log.warning(
-                                f'syncing fsp to source by offset: {diff}')
-                            history = dst.array
-                            dst.push(history[:-1], start=src._first.value)
-                        else:
-                            log.warning(
-                                f'large offset {diff} re-spawn ongoing?..')