Darren,
It sounds like water starvation to me, have you cleaned the filter at the bottom of the fresh water tank?
If this block up its not that noticeable when using a hand tool with a single jet, Pressure drop becomes more notticeable when you attatch a twin jetted floor wand!
Failing that check the hose from the bottom of the tank to the water pump make sure this is not callapsing with the heat of the water!
Its deffernetly a blockage you have somewhere in the water system before the water pump!