I have a RB951-2n for more than a year.
I have 50/50Mbit bandwidth, after random while my speed drops to 10Mbit (1Mbyte) from LAN to the internet or internet to LAN (both upload/download). Connection speed between devices in LAN are not affected. I have graphs for CPU and RAM usage of the router, CPU is less than 5%, RAM is around 50%.
If I unplug the UTP from WAN port and replug it right away the speed goes back to 50Mbit.
I called my ISP, they said everything is fine from their side.
Any ideas?

In each interface you can view the status tab for bad packets and CRC errors.

For the RB951-2n specifically, don't use port 5. In the few I implemented, port 5 always had dropped packets due to CRC errors.

Check your wan interface to see if it's 100 FULL duplex, and not half duplex.

Finally you can use packet sniffer to capture packets. You should see the two end points trying to communicate as fast as possible, and only with lost packets do the end points slow down the connection. With my ISP, "no problems" might mean they can ping the modem. I have to fight tooth and nail to get a "Level 2" technician to run tests to prove packet loss is between ISP and Modem.

Thank you very much for the quick answer!
It isn't port 5.
Do you use Auto Negotiation for duplex speed or set it manually?

I use auto-negotiation unless there is a problem. I've found auto-negotiation always works well with two gigabit end points. But if one or both are 100mbit, then it's possible the the auto-negotiation features are implemented slightly differently, and auto-negotiation may fail. With ROS version around 6.30 and up, the log will show "Excess packet loss or duplex mismatch". Then I set duplex to 100 Full.

You can try setting your want o 100 Full, see if the issue goes away. If it does, it's fine to leave it at 100 Full.

If a gigabit link over cat 5e shows frequent link ups and link downs, then I set the link to 100 Full.