problems. BUT, I'm at a loss to explain the output spectrum.

Around 18 Khz, there is a peak. Exactly 15 Hz below and above the peak
is a smaller set of peaks, presumably these are the sidebands that the
receiver recovers. BUT, there is a peak every 60 Hz that slowly gets
weaker as the spectrum moves away from the (center) largest peak. Each
one of these smaller peaks has upper and lower sidebands as well.
There are many of these peaks, not just a few.

This may be caused a lack of smoothing of the supply voltage (assuming you
are in the US). Lots of peaks suggests FM modulation.

Wim