The float system is not the best design. Pull your first aid kit out to expose the sender, just unplug it and unscrew it and pull straight up without bumping the sides. Perhaps you will see something that is binding. Be careful as it is delicate.
Perhaps the problem is algae. If it is quite black, it means it is coated with algae-and so is your tank. Sterilize the sender with Clorox, rinse, dry and reinstall. This may cure the pulsating. Use a diesel algaecide for the tank.

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