This might not be accurate because of my faulty memory, but it reminds me of someone's problem with a retrofit of, I think, cruise control, during the early ABS years. The sender in the diff needed a frequency divider between it and the speedo to provide fast enough speed info for the braking system, yet match the existing scheme with cruise and speedometer.

But like you said that would not explain the meter working so well unless the exact multiple input locked the phase comparator for the analog speedo output with, say, every 5th pulse.

87 sounds too early for this, even for the cutting edge 700 series. And fixing it with a replacement cluster makes it sound more like a problem in the original speedo's signal conditioning, or odo motor pulse decoupling, delivering a crappy waveform to the chip's input. Even at 5x, that would be noteworthy accomplishment getting a 700 to 2 million.
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Art Benstein near Baltimore