Mechanical Deformation
What about this possibility.
Pumping high volumes (caused by demand for thrust increase) of very cold viscous fuel (fuel still within specs of course), where some part of the fuel system plumbing (between the tanks and the HP pumps) starts to deform and narrow due to the suction presssure of said cold viscous fuel, thus creating a temporary restriction.
In other words, some part of the pipe work partially (and temporarily) collapses due to the suction pressure and the viscosity of the fuel, causing a restriction. This theory is not unheard of, but does require the pipe work to rebound after the suction pressure is removed (thus leaving no evidence of a restriction). This might also explain how it could happen on both sides of the airplane.