sudden power loss, noise
 

I have a factory reman 6bt in a Bluebird bus/rv. It starts and runs well, but I have had it to act like the wastegate is dumping open and sticking. It seems to do this more when the engine is warm and outside temp is high (90+). I do not have boost or egt gage, but everything on the engine appears completely stock. I did check the wastegate actuator with a regulated air source, and the rod starts to open in the low 20s and max travel is at 30 psi. I disconnected the rod, and the wastegate lever moves quite freely and the rod at 30 psi is right where the lever would be in the full open position.
What I experience is a sudden increase in noise (sounds like a huge amount of air), more of a roaring sound. At the same time, the power seems to drop a fair bit, but it will still accelerate. I see no smoke and the engine temp does not seem to change.
It happens under moderate acceleration, or just holding speed on the highway. The bus is like a brick, and runs 60+ mph @ 2400 rpm. The engine sounds reasonably happy at that rpm, but I don't know what the boost pressure is.
Usually, after 10-20 seconds, the noise cuts out and power is restored. It does not seem to matter if I slow down, speed up, or maintain speed when it happens.
I really appreciate any input. I suspect the wastegate actuator, but it works like it should when I can test it cold.