In trying to diagnose why an in-wall ethernet cable would drop from 1Gb to 100Mb but still work. I noticed for a period after plugging in a cable tester there was stray voltage faintly lighting up a couple of LED on the tester (leds 1&2), the stray voltage is no longer there so the voltage appears to be intermittent.

Is this likely induction from the mains? Whats the best way to diagnose this?

Would this be introduced from wrapping around a power cable or just running along side?

Things to note:

I've only seen this stray voltage once.  Nothing was plugged in either end, other than one end of the cable tester, the cable tester remote without power.

Ethernet speed drops to 100Mb from 1Gb, many times.  I now assume this is from the stray voltage.

There is another ethernet cable running along side it, which it is not having and issues, or at least not I have noticed, as the TV is plugged into this is only 100Mb.

The cable tester stray voltage lighting up the tester is faint at one end and very faint at the other.  I guess that means the stray voltage is introduced at the end with the brighter light. The furtherest end away from the switch.

There is nothing switching on and off that would introduce they stray voltage.