This is NOT a "routing" problem. It is a problem with autonegotiation between the Routerboard(s) and the switch(es). I have experienced same problems with (many) RB 493 AH and RB 433 AH. After some time and without serious traffic load, suddenly only the Tx seems to work, while Rx is stuck in zero (0). It makes no difference if you disable/enable the ethernet port in Routerboard (I have repeated more than 50 times this method in each rb). It makes no difference if you change port to the switch. It makes no difference if you connect the "crashed" ethernet port to ANYWHERE (another routerboard, switch, ethernet card etc). The specific Routerboard ethernet is simply dead (!!!). All the other ethernet ports are working ok, unless if you try to connect them to the switch where they also "crash" after a few hours. ONLY if you restart the Routerboard the "crashed" ethernet port goes back alive, until the next crash.
Just wanted to add our experiences.....
We are having a very similar issue. We have RB493AHs that we have been using since it seems we cannot get daughterboards for the RB600s (which have had NO problems for us on probably 30 towers or so). We are experiencing the port lockups, but even more, about 75% of the time that one of the ports lock up the whole router becomes "locked" and a reboot is the ONLY way we can get them stable again. Here is what I have found to cause these lockups:
Plugging in a new piece of equipment (even my laptop to any port)
Rebooting an existing piece of equipment that is plugged into the router
Re-inserting a plug
Now, as these things do not always cause a lockup on the port or router, it is more often than not, and any for that matter is too much in the field. Sometimes we can reboot equipment with no problem- the next day or week rebooting the same equipment causes either the port to lock up or the whole router.
Some have reported that using lower voltage PSUs (other than the 24V we are using) solved the lockup problem, however, this did not help us even when we used 12V PSUs.
Hopefully this problem is solved quickly as we basically cannot use these at all with this problem.