Problem with cisco switch SG500

We are using switch of Cisco belong to series SG500. The current, we saw some of the problems after use the switch (detail below). Please support for us.

Here is a desciption of the problem
-When the switch is started, it runs very normally. The devices are connected to the switch, the switch will learn the MAC address. The number of MAC address learned on the switch about 2000 MAC.
-After switch operate about 2-3 weeks, we found that the number of MAC address learned on the switch reduced more than 80 percent, from 2000 MAC to around 300 MAC. The devices are being connected on the switch still operate normally. However, when we move the network cable of device is using from one port on a switch to another port, we were experiencing long delays in reconnecting to the network. The delays are about 10 minutes.

Detail test after switch operate about 2-3 weeks:
- First device configured with a static IP of 192.168.1.2 is plugged into port 2 of a cisco SG500 switch.
- Second device is configured with a static IP of 192.168.1.3 and is plugged into port 3 of the switch.
- On both device, We start a ping <IP of other PC> -t
- Then we moves the network cable from port 3 to port 4 of the switch. As soon as we moves the cable, pings on both device stop working, check the switch is not learned MAC of second device. After about 10 minutes, the pings start working again. But the switch is still not learned MAC of second device.

Here is detailed information of switch that we are using:
SG500-28 28-Port Gigabit Stackable Managed Switch
- PID VID: SG5000-28-k9 V02
- Firmware Version (Active Image): 1.3.0.62
- Stack unit mode: Standalone
- System Operational Mode: L2 Mode
- Jumbo Frames: Enabled
- Spanning tree protocol: MSTP
- SMNP : Enabled
- Port security : Disable
- Qos: Disable
- Access control : Disable
- DHCP : Disable

Looking forward hearing from you soon.