Wireless router's ping time DI-624 got worse over time

My wireless router is DI-624 and I am doing pings from a wireless client.
The wireless setting is as follows:

WPA-PSK enabled
Disable SSID broadcast
Disable Super-G
802.11g only
All other wireless settings are default.
My internet connection is maintained by this router.

DI-624 Rev B Firmware 1.28
Packets: Sent = 31, Received = 29, Lost = 2 (6% loss),
Minimum = 1ms, Maximum = 2706ms, Average = 725ms

As comparisons, I pinged a local computer and another router

Windows 2000 Professional Service Pack 4
Packets: Sent = 22, Received = 22, Lost = 0 (0% loss),
Minimum = 1ms, Maximum = 7ms, Average = 1ms

SMC7004ABR Firmware 1.40.018 (DHCP is turned off)
Packets: Sent = 35, Received = 35, Lost = 0 (0% loss),
Minimum = 1ms, Maximum = 992ms, Average = 30ms


My internet connection was very slow (about 4.2 seconds instead of the
usual 0.2 second to an internet server I usually use). I wondered if my
router
is the source of the problem. It has been without reboot for a few days.

After rebooting DI-624, the latency becomes so much less.
Packets: Sent = 34, Received = 34, Lost = 0 (0% loss),
Minimum = 1ms, Maximum = 5ms, Average = 1ms

My internet connection becomes normal. The router was the problem! But why?