I experienced a similar issue. But not every 20 seconds, it took longer, but can't say how long. No matter which sound I used, also without sustain pedal it happened. So it was not an issue with polyphony. But anyway, it was annoying and I wouldn't like that in a live situation.

Seems like it was only happening when my (white) Montage 7 was connected to my MacBook Pro (M1) via USB. Without the MacBook I haven't had this issue.

So I tried to figure it out and it looks like it was happening when the computer was going to sleep mode.
But since I use the new driver version 3.0.5 I didn't experience it any more (hopefully it won't come back again).

I've had (and continue to have) this issue on MODX whenever an iPad is plugged into USB, no matter what that iPad is doing.

I'm not sure the frequency of this, but it's certainly much more noticeable when messing around with the keyboard than when playing (via hold) with arpeggios, so it might have something to do with the message from direct key presses getting lost. Haven't confirmed this, though.

Since noticing that it's the iPad/USB connection, unplugging that and restarting the MODX makes it stop happening.

At least one other person has had something similar, and it's mentioned here:

https://www.yamahasynth.com/ask-a-question/usb-audio-problems

Thanks for the responses thus far. I use a PC with Cubase and nothing else. Notably, I haven't yet had this experience when I'm recording in Cubase, but it could just be a matter of time before I do. If that starts happening, it will be impossible to record while experiencing this problem....

The problem occurs when the USB cable is disconnected from your computer?

Since others have mentioned the possibility of issues with USB of the host system going into low-power modes I'm wondering if there is any relation or if you see the issue (@David) when there is no host involved and completely disconnected.

If this only occurs with a computer connected - then there are a few strategies to ensure the host system does not power down a USB port and that the host system itself doesn't go into a sleep mode.

If your sound is intermittent and there is NO COMPUTER involved, you should seek help from an authorized service center, as soon as possible.
If, however, it only occurs when connected to a “host” device (computer, laptop, tablet or smart phone) then your issue is related to your host device and/or the connection to it.

Is it the synth? Is it the “host”? — This part should be easy to diagnose. But tracking down the exact cause requires you methodically run checks on your computer/host system and connections.

Problems occurring at regular intervals is surely a problem related to the host.
Problems occurring occasionally could be any number of things (including a bad connector on the synth - so we cannot eliminate it entirely).

Important: When you have a good solid USB connection, the USB icon on the very top line of the screen will be bright blue and steady. If it is intermittent (flashes off and on), this indicates a break in communication (problem).

At that point, power down both the synth and the host.
Reboot everything and attempt to reestablish the connection.

If you are having an issue where the sound intermittently stops, immediately check to see *how* you are monitoring the signal. We will assume everyone is using the MONTAGE as the audio interface. Switch to “Direct Monitor”. [UTILITY] > “Settings” > “Audio I/O” > set Direct Monitor = On. If this fixes it, let us know.

Extra Credit
Cables are more likely to be a problem than the synth itself.
Adapters are more likely to be a problem than the synth or the cable.
If the synth has an audio intermittent problem, it would likely appear whether connected to a “host” or not
Your computer or host is more likely going to be the issue.

If you are connected to a computer:
Make sure you are using the Yamaha Steinberg USB driver appropriate for your computer type and operating system.
If you are connected to a tablet or smart phone make sure it is in Airplane Mode, and has enough battery life to function normally. If you do not get the steady blue USB icon on your synth’s screen, reboot your host device to help it reset.

Great information and a few things for me to check on.

Regarding Important: When you have a good solid USB connection, the USB icon on the very top line of the screen will be bright blue and steady. If it is intermittent (flashes off and on), this indicates a break in communication (problem).

"the very top line of the screen" refers to which screen?

Thanks.

the very top line of the screen" refers to which screen?

all screens, literally, all screens.
You’ll see the “FX” icon, the Quick Setup icon (keyboard and USB/MIDI connector), Tempo (quarter note and value), Live Set grid icon, Gear icon (takes you to the most recently visited Utility screen).