Only listed routers are affected by this issue: RB450G, RB750G, RB435G, RB493G
Certain RouterBOARD products tend to become unresponsive for some amount of time, after a while becoming available on the network. Similar problem is - that only guest becomes unresponsive and after a while continues to perform as expected. If watchdog is enabled on the router - router will be restarted by it instead of becoming available on its own.
Other routers from this architecture are believed to not to suffer from this issue.
Other issues that do not fit the description most probably are caused by RouterOS misconfiguration and does not have common denominator and have to be checked case by case.
As alternatives listed boards can be used instead: RB2011, RB433AH or any other from these series