2005 W211 E320 CDI Power Steering is getting weak.
 

2005 E320 CDI, W211, 170,000 miles. 4-door sedan has speed sensitive steering. Lately is appears the car has less power steering boost at speeds of 20-45 mph, especially at slow engine RPM. It seems to have normal power steering at very slow or stopped. At slow road speeds the steering seems stiffer than normal. If I manually down-shift the transmission and get the RPMs up to 2000 or so it seems better. The reservoir level is ok. If I look into the reservoir while idling it appears the fluid is not swirling. There are no error codes logged or at least detectable on my cheap reader. I'm thinking perhaps the screen/filter in the reservoir is getting clogged. Any suggestions on what to test or look for? Thanks.

This could be the issue you are facing.

http://forums.mercedesclub.org.uk/in.../#post-1618096

I put pentosin chf11s but it didn't fix it. I changed the PS reservoir last week and put MB power steering fluid in it to no effect. At low speeds the steering gets very stiff.

https://cimg1.ibsrv.net/gimg/www.mbw...2c724c1b8d.jpg
https://cimg2.ibsrv.net/gimg/www.mbw...9a4028f7d2.jpg
As referenced in link previously provided consider having solenoid valve (part # 2114600984, 210 in diagram) @ steering gear replaced. If replacing steering gear (210) is recommended shop 2114602000 replaced by 21146003200.

Thank you for the ideas. I inspected the outside of the SPS valve and connecter from under the car and everything seemed intact and ok. I think I'll speak with an independent shop I trust and see what they think. The problem seems to have started when the winter rains began.

Change the old fluid.