Outer Limits 4x4 Board

When driving quickly steering from full lock to oppsite full lock. I loose power when i start to turn too oppsite lock. Can be very scary inbetween trees.
Would this be a power steering pump or power steering box problem? How would be best to diagnose the problem.
Or this a commmon problem? If so how can it be rectified?

Nissan patrol GQ 1990, rb30 engine. GU power steering box and 35 tyres.

id be leaning towards sticking bearing inside the pump wheel, so when it needs the power it bogs the engine as thats the only part that can have a direct impact on the engine itself ie loss of power.

Sorry.
i ment i loose power of the steering.
does that help ?