Turbine throttle goes up and down

[brygun]

Going back to SSTO/ASTO Kerbal projects. Watching the climbing flight the throttle is down and up on its own.

I am using Mechjeb and I see this with Mechjeb turned off (still installed, just not using the nose pointing function)

I thought maybe it was tied to air intake so added more. Persists with Kerbal Engineer is showing like 1 unit demand to 9 unit in supply

MechJeb Vessel Info did allow me to track the possible vs current thrust. At time my possible was 13 m/s2 while the current was going up and down on its own to as 9 m/s2 (a huge power loss for something trying to hit mach 3!). The vessel's throttle was going up and down in mach 0.8 - 1.5 range at 5,000 to 15,000 m. The altitude was changing so the air pressure would to.

 

What is it making the throttle go down and cut power like that? Is that normal?