fetching cdp from rra -- Win7 64bit x86 fail

I've built trunk under Win7 64 bit with x86 as the target platform, and 
am using the dotnet bindings to access rrdtool.

I have a simple pair of apps: one that writes metrics to a rrd, and one 
that plots charts from them. Each loops indefinitely, repeatedly writing 
and plotting.

The plotter fails quite consistently; it always fails, but the time to 
failure varies. The fail always appears to be "fetching cdp from rra", 
indicating it's occurring in rrd_fetch.c, having failed to read the 
expected number of bytes. After the first fail, the plotter repeatedly 
fails until the process is restarted.

It feels like a problem with built-up state, but I've not identified 
where it lies.

Is this a familiar problem?
Is this model (looping, rather than one-off calls) sensible?

(Am happy to post example code if useful)

Thanks,
Tim