I have two RB2011L's and each have the same problem. Randomly I will see some of the ports lose their link state. Ports 1 and 2 which are Gigabit will lose their link state and then other times port 5 will do the same. There is no loss of power, the cables are good and I have had not had these issues with the RB750's and 751's.
The first 2011L was installed in April and the 2nd one last month. They are both using 5.24