Are you running filters in the filter housings?  If you are, which mesh?  We normally run a 10 or 30 mesh with fertilizer.  I have seen the pump overspeed when a fine mesh filter is used with fertilizer.  The fertilizer plugs the filter and limits the flow so the pump ramps higher.  With the pressure transducer behind the filter, the pump doesn't know it's getting into the danger zone.  But, in all reality, a course strainer will only HELP because, like Lil' Hoss said, there were only problems with a few machines that had a bad batch of filter housings.