Symptoms: intermittent and frequent loss of internet connectivity. I could ping the wireless router but not the DSL modem. Windows 7 Professional.
Problem found : an inappropriate persistent route that had been configured into the system by a misbehaving application (antivirus, vpn, that type of thing).
Solution: disable all interfaces, use route delete command to verify no persistent routes no longer in effect, reestablish connection.
No problems since.