John,

I believe your problem is the bridge recitifiers.  A bridge rectfier circuit
produces  floating outputs.  What you need to use is either a half or full wave
rectifier circuit with the ground attached directly to the transformer winding.
This will keep the ground from floating and keep your power supplies from
tracking each other.   If you can arrange your transformer secondaries into a
center tap configuration, then use this center tap as the ground.  Once this
grounded center tap is in place, use full wave rectifiers to produce your
positive and negative supplies.

----- Steve