Problem with nearly new Nucleo F722ZE

[TLanc.1638]

New Nucleo-F722ZE initially worked fine (initially prompted for software update). But after approx 3 hours when trying to Run->Debug , it now shows:

Waiting for debugger connection...

Debugger connected

Debugger connection lost.

Shutting down...

Power cycling everything doesn't help. Tried re-installing the latest ST-LINK software, still no change.

Nucleo-F429ZI, works ok on same computer and USB port.

Is the Nucleo faulty, or is there something else I can try?

[Tesla DeLorean]

Using a DELL?

Watch for Diagnostic Agent or Intel performance drivers in the mass-storage stack.

Check for screen saver or hard-drive power down on the PC.

If your code puts the device into low-power mode, ie WFI or SLEEP, it powers off the debug interface.

Watch also for disabling or interference with the debug pins.