1998,Power steering is hard, softens when I accelerate

Power steering is hard when the car is still then easens once I step on the accelerator. I have replaced the power steering pump and once the engine is running once can see the steering fluid circulating when you open the cap on the power steering reservoir. About two months ago I also replace the steering box. The car is pretty old so I get parts are getting to the end of their useful life, but at the same time its frustrating as it now has a new steering pump and steering box. The pipes have also been replaced. Will really appreciate your input on this.
My car has a manual transmission.