Hi @enderocean,
I doubt the temperature would be an issue unless it was incredibly hot (like in an oven kind of thing). I’d recommend you check that all the relevant connections are correctly plugged in at each end (e.g. the wires to each fathom-X, the ethernet cable between the fathom-X and companion computer RPi, the cables between the top fathom-X and the surface computer).
@sample’s suggestion of checking the slip ring is another good one, although I’d recommend doing the connector and cable checks first since the slip ring is quite delicate, and can also be difficult to put back together once it’s disassembled.
Our tether technical details specify the electrical resistance at 20°C should be 0.127 Ω/m, so for a 300m tether you can expect that the resistance of each wire is 300 \times 0.127 \approx 38.1\Omega. You should be able to measure that with a multimeter on the connectors that go into the Fathom-Xs (you only need to check the two wires in the blue/white pair since that’s what’s used for ROV telemetry and comms). Given your connection is intermittent, you might want to try measuring resistance while moving the cables around a little to see if the resistance suddenly spikes.
If you have a spare (unused) pair in your tether it could be worth swapping to that and seeing whether the connection issues go away, which would simultaneously provide a temporary solution to your issue while also confirming that the issue is indeed somewhere along the blue/white tether pair 