I've been on the same trouble for almost a year using a 493AH, that I tried to replace with a 493G with no success (instead it went worse, so I reinstalled the 493AH). On the other end I had 3 RB433AH. No matter in which port 2-7 I had them connected. I also installed new cabling from the same provider I had already installed.
Well, I had two issues:
- Permanent: no 100mpbs full-duplex link using about 3 meter of supposed cat-5e/cat6 cable. Link sync changing constantly between 100FD right after reboot, 100 Half a minutes later and even 10mbps!! (FD/HD depending on unknown conditions).
- intermitent: loose bidirectional connectivity on the ethernet.
- once: total loose of connectivity, even with electrical reboot of the boards.
Last week I replaced everything for a couple of 450G and nothing changed, still had the first syntom: 100 half duplex.
Then I changed the cabling and connectors to two RB433. I used an apparently similar cable but from different provider and metallic UBNT connectors with the ground wire attached on both ends.
Everything worked like a charm since then. I still have to change one cable.
The cable I used was 4 twisted pair, 24AWG solid copper with ground wire.
Try using a different cable