One caution about lowering the relief pressure:
The designers may have designed the system such that the engine stops, instead of the pump going into bypass. All of the power generated by the engine is turned into heat as it passes through the relief valve. Normally that power(heat) is dissipated into the brush you are destroying.
If you put the system into bypass for a relatively short period of time, it will overheat the oil, and likely destroy any seals in your system. If the engine doesn't have the power to put the system into bypass, the relief valve only needs to relief impulse flows, which are very small.
Also, lowering the engine RPM probably won't help your problem. The shutdown is caused by the pump requiring more torque than the the engine can produce. The required torque for a given pump is only related to the differential(discharge) pressure. Since the torque curve for a diesel engine is relatively flat, changing he rpm should not have much effect.
I guess what I am saying is be careful. If you are the one driving the machine, and know the issues, it may not be a problem, but if you are hiring someone to run it, and they constantly are putting the thing in bypass, it will likely break.
Chris