problem with my wireless card inspiron 1521

Hello! my name is nick! I am experiencing some troubles with my inspiron 1521's wireless card! I am conected to a local router witch provides me internet via a broadband connection(wan miniport pppoe)....two days ago the broadband connection failed...and recovered after i put windows to repair the wireless connection! It worked for a few minutes and failed again...i did the same thing and the connection recovered!!! The strange thing is that only the VPN(broadband connection) fails , the wireless connection to router is entirely fine!!! Since then i can' t stay conected to the internet longer than 3-4 minutes! I mean through wireless connection...because if i connect to the router with a cable(LAN connection)...the broadband connection and the internet work just fine!! To write this message i pluged a cable into my router... ...and i'm using the lan connection! The other guy who uses the router with wireless connection has no problems! His wan miniport (pppoe) connectoin is workin absolutely normal! I tried to reinstall the wlan mincard 1390 driver...i did it, but the problem is still there! I forgot to say that the first time i experienced the problem i was using a windows xp sp2 os....i've uninstalled him and i installed windows vista ultimate (dell lite)...it worked fine a few hours but the problem reappeared...same problem! My notebook connects wireless to the router without a problem but it can't connect to the internet using a broadband pppoe connection... if i connect him to the router with cable, it connects absolutely normal to the internet using the same broadband connection!! Any advice will be well recieved....thanks!!!!