Gold Trail,
Sodium thiosulfate is used as fixer in the photo industry to dissolve silver halides. The silver is then plated out in special electrolytic units. The cathodes in these units are usually stainless steel drums which rotate. The purpose of this rotation is to provide fresh silver ions directly to the cathode surface. The silver in this matrix is in the form of a negative complex thiosulfate ion. This negative ion is actually repelled by the negative cathode - this is important to understand. To break down and deposit silver, these ions have to virtually be next to the cathode surface. If the cathode surface is depleted of these silver ions, the cathode will instead deposit a black sulfide deposit, created from a breakdown of the thiosulfate. This is the reason for the rotating cathode - to keep the cathode film (an extremely thin layer of solution right next to the cathode) full of silver ions and prevent sulfide formation. It most probably works the same with gold in this system.
In this silver system, the black sulfide also forms when the silver concentration gets low. Same reason - there's not enough fresh silver ions next to the cathode.
I don't think that normal agitation would work, unless it were extreme. I think the rotating drums agitate the cathode film, whereas normal agitation would primarily agitate the solution. The cathode film is actually a separate entity from the body of solution. To replenish the cathode film with negative ions, you might be able to use ultrasonic agitation. I've also thought that the solution, circulating through under-the-solution spray nozzles, which impinge directly on a sheet cathode, might work.
This cathode film thing is a complicated subject, which I don't understand thoroughly.
