Internet dis-connects

Have a wi-fi, only one device is affected:
MacBook with 10.5.8

Upgraded to 10.6.3 to no avail.

Network connection panel say connected but no browser can access a url.

Happens only after sleep, and not always.

Only a re-start works for getting connected again.

Tried resetting keychain. Tried deleting 'Preferred Networks'

Any thoughts appreciated.