diff --git a/piker/fsp/_engine.py b/piker/fsp/_engine.py
index e246e0b7..0494ef3c 100644
--- a/piker/fsp/_engine.py
+++ b/piker/fsp/_engine.py
@@ -123,20 +123,11 @@ async def fsp_compute(
     #   is `index` aware such that historical data can be indexed
     #   relative to the true first datum? Not sure if this is sane
     #   for incremental compuations.
-    dst._first.value = src._first.value
-    dst._last.value = src._first.value
-
-    # compare with source signal and time align
-    # check for data length mis-allignment and fill missing values
-    diff = len(src.array) - len(history)
-    if diff > 0:
-        log.warning(f"WTF DIFF fsp to ohlc history {diff}")
-        for _ in range(diff):
-            dst.push(history[:1])
+    first = dst._first.value = src._first.value
 
     # TODO: can we use this `start` flag instead of the manual
     # setting above?
-    index = dst.push(history) #, start=src._first.value)
+    index = dst.push(history, start=first)
 
     profiler(f'{func_name} pushed history')
     profiler.finish()
@@ -146,8 +137,8 @@ async def fsp_compute(
         task_status.started((cs, index))
         profiler(f'{func_name} yield last index')
 
-        import time
-        last = time.time()
+        # import time
+        # last = time.time()
 
         # rt stream
         async for processed in out_stream:
@@ -254,12 +245,12 @@ async def cascade(
 
                 async for msg in stream:
 
+                    # respawn the compute task if the source
+                    # array has been updated such that we compute
+                    # new history from the (prepended) source.
                     new_len = len(src.array)
-
                     if new_len > last_len + 1:
-                        # respawn the signal compute task if the source
-                        # signal has been updated
-                        log.warning(f'Re-spawning fsp {func_name}')
+                        log.warning(f're-syncing fsp {func_name} to source')
                         cs.cancel()
                         cs, index = await n.start(fsp_target)
 
@@ -268,8 +259,22 @@ 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()
                     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?..')