Linksys Wireless Router :: WRT54GH Frequent Disconnection With DSL Modem

I purchased WRT54GH wireless router back in 2009 and it was working fine except for past 1 month, wherein the Internet Connection was being disrupted 15 - 20 times in an hour. I thought it could be fault from ISP end; however after investigation by ISP we found the fault was not from ISP as, when Wireless was unable to establish connection with the DSL Modem it was failing but when tried to connect laptop with the DSL Modem it worked fine (Wired connection). I did try the Settings mentioned in an earlier post to change the UPnP and the Beacon Interval etc. This however doesnot seem to work as the internet has been disconnected almost thrice since then. (Approx. 15 - 20 mins). When the router is unable to connect to internet I restart the Wireless Router and the DSL Modem. After restarting, the Internet connectivity works fine.
 
Below are the Advanced Wireless Settings from Wireless Tab:
 
Authentication Type: Auto Shared (Default: Auto)
Basic Rate: Default (Default: Default)
Transmission Rate Auto (Default: Auto)
CTS Protection Mode: Disabled Auto (Default: Auto)
--------------------------------------------------------------------------------
Beacon Interval: (Default: 100 Msec, Range: 20 ~ 1000)
DTIM Interval: (Default: 1, Range: 1 ~ 255)
Fragmentation Threshold: (Default: 2346, Range: 256 ~ 2346)
RTS Threshold: (Default: 2347, Range: 0 ~ 2347)