162.4 my noaa "test" frequency for overall checks.
Excellent, thank you for that! I'll check out that spot frequency in the lab tomorrow. There may be a splat of internally generated wideband noise around that area, appearing to desensitise the receiver... will find out.
I previously measured my 878s every 5 MHz between 130 & 180 MHz and plotted the results.

As you can see, no hint of a problem there, 0.13uV for 12dB SINAD at 160-ish MHz with a mute threshold of 0.1uV is pretty good - and things only improve closer to the ham bands. It's the same story with another two 878s I tested too, all turning in the similar results. What puzzles me is that the HD-1 and the 878 use the very same RF receiver IC, the ubiquitous AT1846S. In theory, there should be minimal difference. In practice, the 878 ought to have the edge given its four pole varactor tuned front end filter, meaning the front end should be able to better cope when hit with strong out of band signals.
Unless there's a wideband noisy spuglie around 162 MHz, it could be that there's an issue with the varactor tuning - either component failure, or incorrectly tuned / software corruption of the tuning values. I'm wondering if repeating the test with a steady but weak signal in the ham band would give the same result?