Hi van jagatha,
I appreciate your efforts to fix the WiFi issue on your laptop. I suggest you to change the Roaming Aggressiveness settings and check if the connection remains stable by following the steps provided in the link below:
http://intel.ly/9bj4Vf
You may also run the online diagnostic test on the wireless card from the following link:
http://dell.to/We70YO
Please let me know about your findings.