Yamaha CR-820 Tuner Issue (Tech input sought)

I have a vintage late-70s Yamaha CR 820 receiver in overall excellent condition. It has recently developed a serious issue with the FM tuner, and I’m reaching out for ideas as to what may be wrong.

It’s currently at an electronics shop, and while I know them to be honest and hard-working, I’m not sure they’ve zeroed in on the problem. They’ve had it over three weeks now.

So here’s the issue. Everything on the receiver was working great and then suddenly the FM sound just dropped out. The tuning meters still work and show when I’m on a station, but there is no sound from the FM tuner. The FM does NOT work, the AM WORKS.

Here’s another twist, if I let the receiver sit idle for 30-ish minutes, when I turn it back on the FM will play for 5-to-60 seconds, and then cut out silent. If I immediately turn it off and back on, still silent, But if I let it rest, it will play for a few seconds as described above.

Open to technical suggestions. Thank you in advance.
The tuning capacitor may need to be cleaned. There are videos on the web showing the procedure. It’s made improvements on all the tuners on this end. Outside of that...... Good luck. 

RadioXtuners, maybe?