I have a gas powered gen with 20 hp honda motor, 30 min after start up the amp output decreases, way low. At first start it is great, any ideas?

Assuming engine is running at full speed, look for heat causing a bad or open electrical connection some where. If letting generator cool short period restores operation, look for heat created by the electrical system causing an open circuit. If generator must cool completely to restore operation, look for engine heat causing an open circuit in wiring.