Dave I guess once the water flow is restricted then the engine struggles.
I still don't get it though.
The pump would struggle, yes, but it has a built in bypass to prevent a too high build up of pressure.
What about when you lay down the wand or stop triggering, there no water flow then but it doesn't affect the engine.
If the problem has been cured simply by de scaling, then obviously that must be it, but seems a strange one to me.
Dave.