[NoDOHC]

Just a question: Are you running a returnless fuel system, because if not, the fuel flow rate is fixed by the pump, it either goes in the injectors or through the FPR, but it always flows the same from the pump at a given fuel pressure. With the engine off, the pump should see load equivalent to 0 psi boost. Under boost, the pump has to make 13.75 psi more fuel pressure.

I think that you are on to something, but maybe not exactly as you described it above. I think that your fuel pump is overheating, which is decreasing the available fuel throughput of the pump and is starving your engine for fuel. You can boost for short spurts just fine, but the pump can't handle the 57 psi for very long without drawing more current.

One more thing, it is likely a multi-stage impeller-type pump, which means that under boost (when the pressure is higher) the flow will actually decrease due to limitations of that design, without factoring in the change in coil resistance due to temperature. (Look at turbo-map speed lines for an idea of the curve shape).

You should have a fuel pressure correction map in the Motec... If not, you can set up an inverse trim to the 0-5V input. Maybe that will avoid going lean in the future.