I'm running into a strange issue with a new flow graph  I'm working on that uses a custom C++ block  that I built. The block takes input samples, performs some DSP, and outputs modified samples at the same sample rate.
When the flowgraph has a single instance of my custom block enabled, it works fine. But when I enable a second instance of the block in a separate second usage, (think stereo sound, one per sink channel) it acts very unexpectedly - output samples are wildly inaccurate and distorted.
My perception is that the two instances of the same block seem to be sharing some part of their memory namespace and as they modify/store variables they are doing so in some sort of shared namespace or memory segment.
Is it possible for two blocks to corrupt each other's memory? I can't think of any other explanation for the two streams working fine individually, but when both are enabled at the same time the output is garbled. I've tried all sorts of things like tying them to the same source block, removing the upsampling/downsampling, adding a delay to one, etc. with no luck.
 "Flowgraph with two parallel streams both using the custom block pq_rails": https://github.com/regulatre/gr-powerquality/blob/master/examples/streamer.grc