CC2420 Receiver Problems

I'm testing a CC2420 PCB design with an Anaren balun that has nominal transmit output power, performs CCA and ED properly, and has a good EVM. However, PER rates are too high on channels 12 and 13 (>1% @ -85dBm), while the PER on other channels is less than 0.1%! We also have an adjacent channel rejection problem on channels 11 and 22. Are these symptoms consistent with a demodulator problem? What might cause this?

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