I thought I'd post this for interest to others, and because I'm open to suggestions.
Whilst trying to work out where the intermittently high error seconds and occasional loss of synch were coming from, I wandered around the house with an AM radio.
At around 700Khz I found a VERY loud buzzing sound. Now and again it would rhythmically drop out with an almost morse-like pulsing. dit dit dit dit.
Yesterday I blamed the bargain NAS and it's PSU, which I got off ebay and has caused me varying degrees of woe ever since, so I disconnected it.
But today I found the noise louder than ever.
Unplugging my gigabit switch made the sound decrease drastically in intensity, and the pulsing disappeared.
Aha! Switch or PSU faulty / bad design.
Except, wandering around the house, the noise came back. I could detect it everywhere in the house.
I went outside, and it was a lot quieter, but holding the radio to the house wall, I could hear it very clearly.
Up to the shed, it was very loud there too.
Near the phone line, I could hear the usual hiss of ADSL, but no buzz.
It seems that the noise is being radiated throughout the house ring main, and up to the shed via the mains cables, because isolating the shed from the mains made the radio fall silent. (not because the radio went off - it's battery
But the loud buzz was replaced with a faint hiss when the mains was disconnected from the house).
Definitely mains bourne.
And network related, since I could quieten it by disconnecting a network switch.
Near the downstairs PC, the noise was very loud, disconnecting the gigabit switch there made it al but fall silent. Aha! So it's that!
Replaced switch with a 100mbit one I had spare, only faint noise. But gets louder. So not the switch then
Unplug PC (which was "soft" off anyway). Silence.
Plug PC in again - noise.
Power up PC, noise back but reduced.
Switching it on and off causes the noise to vary on the radio in a different room, and as I'm typing it's just started buzzing very loudly again, even though the PC in question is "soft" off.
I've now unplugged the suspect PC, and 700Khz is pretty much silent now.
So, I'm assuming here that the PC PSU is at fault. Radiating noise into the mains and into the network wiring via the NIC.
Fair assumption, or did I miss anything?
Ian