How familiar are you with troubleshooting - specifically, tube PA circuits?
For starters, have a look at all voltages involved with the final. Specifically, screen grid and plate. Be careful. Are any of these dipping as you transmit? And when looked at via scope, are they clean, stable and free of ripple?
Next - possibly gassy tube. Replace with known good part and test again.
Then we'll work backwards to the driver.
