HG,
I have been watching yr progress and progress has been good.
However, your trouble started when you changed the supply from +/-12 Volt to +/- 14 volt. it is not clear to me how you are powering your MPX. From memory, these transducers are specified at 10 volt and are nulled at this supply voltage. They have a maximum supply voltage of 16 volt from memory.
Is it possible you are supplying the MPX from the +14 volt supply? and this value of supply voltage is approaching the maximum?
I have a 'universal' digital readout unit, and this includes a 'LH0070' which is a 10 volt precision reference. This supply is available for driving bridge devices like your MPX.
Its just a thought that maybe you need to look at the supply which drives your MPX.
Hope this helps.