I do not have a schematic for this chassis , but you may want to consider power feed interuption instead of the switched ground fault.
It is possible that you have a poor connection at the fuse feed or relay connection .
Hook a test light to ice cube coil side and load side and see if power is dropping at the terminals of each.
If suspect , bring a bat+ jumper over to ice cube pos side and run the car for a test .. see if it drops then.
Same goes for ground side .. Run ice cube with its own ground wire and see if you get a drop after the time limit you are getting.