Improve power steering when rack and pinion starts binding.

Disconnect the tie rod links to the rack & pinion steering gear, and hand test the front end.
You probably have a binding tie rod or ball joint in the front end. Remember too, the steering pump or the rack & pinion could be bad, creating this problem.
With everything unhooked, the problem should be easily evident.
God bless your efforts.

Hey Kumar,
Does the steering get easier when it gets hot? If it get easier when hot, its the rack and pinion gear. The valve body (actually a teflon composition ring(s) o-rings/valve bore housing are getting worn to the point where they are not controlling the fluid flow when its cold,, when the rack and pinion gets hot, the valve bore expands and "corrects" the clearance which in turn, controls the fluid flow. The valve body bore in the rack and pinion housing is worn where the teflon rings ride against the bore.
This is nothing that can be fixed except by replacing the entire rack and pinion gear. Drain ALL the power steering fluid from the pump also and fill with fresh new fluid.

It is possible that your PS fluid is the wrong type or has broken down. There are differences in the composition, not all PS fluids are alike. Chrysler, for instance uses something different than Ford and the pumps will whine and fail if the wrong fluid is used. This may be true with Mercedes.

Check with Mercedes for specs for your requirements. It is possible that the rack and pinion which moves the front wheels has issues and is either leaking internally or is binding. Binding requires the pump to work harder. The output of your new pump can be pressure tested as well as the rack and pinion.

A transmission shop would have hydraulic testing gauges or Mercedes will. It may be cheaper to try and bleed the old fluid out of the system, front wheel off the ground. Then disconnect line to rack and pinion and work rack side to side to bleed fluid. Then install new fluid and see if it helped.

Maybe the strut mount bearings.

could you have a binding tie rod end or ball joint?  Yes there is adjustment on the rack and pinion at the tie rod ends.