Cyclone IV - timing problem with I/O pins sharing Vref

I think in this case it may be better to punt, and just choose a slower oscillator. Unfortunately, this means physically replacing a part, but we are well over the "minimum" sampling frequency to get adequate results from the synthesizer. The previous incarnation was running fine with a 66MHz sample frequency. Since we have no specific requirement for anything above that, it seems rather pointless to keep fighting the hardware.

On the bright side, this does explain the weird signals I was seeing on my scope. The sign bit (bit 13) is one of the bits affected, and there is at least one other high-order bit (9) with the same problem.