Is this problem manifested by crystalization of copper on the anodes?  On the
bottom of the anodes, right?  Is it more prevalent on some parts than on
others?  Have you looked at the anode c.d., the cathode c.d. and the
resulting anode-to-cathode ratio?  Is it high or low?  Have you tried anode
shielding?  Have you tried different concentrations of sulfuric acid?, (this
crystalization is the "common ion" effect, sulfate being the ion).

There could be several answers but I've seen anode shielding fix it.

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