I'm having similar issues - intermittent low speed heavy steering on an 11 R-design with the speed sensitive steering. Already had the solenoid replaced on the rack, and flushed through twice with the green central hydraulic fluid which has made some difference but still plays up from time to time.
I need to look at where the electronic controller is to find the part number and see if I can replace that next. The pump doesn't seem to be overworked as revving it doesn't make much difference - it seems the brain behind the steering is convinced to reduce the assistance thinking you are moving ...
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