Hello,
I have the following problem with my 850. Currently my car has power steering that operates intemittently. As crazy as this may seem hitting bumps seems to cause the steering to operate or not operate.
I've checked the fluid level and it seems to be OK. Also,I can see any leaks anywhere. My thoughts are that there is an issues with the pump.
Has anyone encountered a similar problem?
Thanks.
Joel

Open the lid on the ps pump when the car is on. Are there any bubbles? That is a sign of an air leak which affects the power steering.
or..
Since you dont see an visible leaks and I suspect no groaning noises since you didnt mention it. I would go with the Steering universal linkage.
It may be binding giving the effect of power steering loss at intervals during the steering range. You can try repacking it with grease but most likely it will need replacement. On mine I brought it finally to shop and he heated it with a torch and repacked it with grease.. been fine so far but I presume it will need to be eventually replaced.