Mikrotik RB951G not pass packets over 35,000 bytes in what could be the problem? For example, on Omnitik pass packets up to 65000 bytes.
IF the routerboard not far from client, reduce the output power from your routerboard.
The devices are connected by Ethernet cables.
Sorry my mistake 
See of the network cable is good.
Network cable changed, used 100 mbps connections. I do not think that the problem is in the cable.
Tomorrow re-check the cable.
ping -n 1000 -l 35000 bytes
80% loss, lost packets 808
ping -n 1000 -l 30000 bytes
1% loss, lost packets 18
The compound of WiFi.
ping -n 1000 -l 65000 bytes
0% loss, lost packets 0
Checked - the cable is good.
support@mikrotik.com not responding