WAN health check is just a simple program sending the health packet and waiting the response from the health check target to determine the WAN conditions. WAN health check failure is always cause by health packets sent but did not get responded.
If you feel that the DSL WAN is good, you can simply isolate the issue by disable the WAN health check (Assuming Health check target defined is stable) . After disabling the WAN health check, you can verify the WAN connection from you laptop, and if you found internet intermittent for sometimes that show that the WAN is intermittent for sometimes and this cause the WAN health check failure.