@johnsa3 There's no issue with the ISP provided router (in modem mode) as I've checked the logs and they are clean. I only get the problem with the router as indicated in my OP.

@terziyski Sound advice, but I suspect the problem is with the router as whenever the problem appears, I get the messages indicated in my OP and never any other time.

@terziyski Already tried that. They sent an engineer and all is fine. I'm running their router in modem mode and all the stats on that device are fine.

@terziyski I've been running speedtests during these issues and although the download speed seems OK, upload speeds are terrible.

I'm having intermittent speed issues which seem to happen at the same time as these messages appear in the log: 2024-04-13 15:57:37 locale[6519]: <6> 283051 Language is changed to 'en_US' 2024-04-13...