Gcmoir
From your description, these surges do not occur during an engine test, such as a DCA test, but in normal running.
It might be too obvious but if these are old 76s, the over speed electrical system should be disconnected. I had several surges when this system played up and had to grap the CB in a hurry.
Whoops!!
I see you did get it in the DCA check, not in flight!