If the motor runs well in 4 low, you probably do not have an exhaust flow issue. The fuel pump pressure is in the ball park, a bit low, but probably not enough to warrant replacing the pump. Did you replace the filter? A 1 psi drop through a filter might show a partially plugged filter. I often restricted (but not stopped) the fuel flow through the return line. Are all your lines and hoses in good shape?

So, now the question is, is there a carb stumble, or an engine misfire? How long ago was the motor tuned up, with plugs, cap & rotor and wires? Most engine misfires occur under load, but they are often present at other times as well.

It really sounds like maybe your Themoquad needs a new accelerator pump, to me, but I've been told that Thermoquads are simply out of adjustment when they don't work properly. Yours must need an adjustment. Maybe someone can walk you through that process. Gene