OK, this is to confirm that the issue is temperature related.
This afternoon, I plugged the LimeSDR in (both a 12V power supply and the USB3 connection) and cyclically ran LimeUtil --make: the LimeSDR replied alternatively with a Gateware version of 0 and 257 (which is not unheard of, see LimeSDR USB Gateware Update Oddities : Version 257?).
Then I just let it rest while supplied for about 20 minutes, and then unplugged and replugged the USB and ran LimeUtil again – now it consistently gives Gateware version 2, revision 16 as expected.
As the computer is always on, its own temperature did not change; only the LimeSDR’s did.
Paradoxically, if I did put a fan on it, it might get worse…
(title updated according to experiment result)
Cheers,
Albert.