Severe Wifi dropping on Macbook Pro - Have to restart to turn back on
I would strongly agree that in most cases, when WiFi signal starts strong after a reboot (when the hardware is up to normal operating temperatures) and then degrades over minutes, it is most likely a symptom that the amplifier is failing and the WiFi card (or router) needs to be replaced.

You can hold the option key down when selecting WiFi in the menu bar and watch the Transmit Rate and RSSI change over time. A repeatable, measurable drop in transmit rate would back up the hardware fault as a cause for this.