XM Steering

[Doc]

After nearly three years of good service my 1992 XM 2.1 TD Hatch has developed an intermittent and in my opinion - dangerous fault. The steering loses all power assistance suddenly and for no apparent reason. [:(]LHM levels are good, filters are clean and the fluid is less than 6 months old. This sudden loss of assistance happened this morning whilst negotiating a right turn at a junction. Needless to say it shook me up as it felt like the steering had locked!![:0]
Afer crawling to a stop, the steering regained full assistance and the car has been okay. The last time it did this was a couple of months ago on a cold morning.
I have checked for leaks and checked pump drive belt tension and condition. All are okay and the belt is only 6 months old. I have heard about pinion valves mentioned on this site - could this be the problem and if so, is it easy to change with engine in situ. I had the engine out last year to sort out oil leaks and a knackered cambelt tensioner. As the steering was behaving itself, I only checked for obvious damage to the rack, gaiters and hoses. I don't want to have to pull out the engine again. That was 9 hours of mental torture and skinned knuckles!!
Any suggestions greatfully received. This has reduced my liking for hydraulic Citroens some what and I fear the laws of diminishing returns is going to apply to any repairs.
Doc

[NiSk]

I would rather think it's due to the hydraulics - I have had the same problem a couple of times on my '93 XM TD12. The first time i cleared it by cleaning out the LHM tank, and running on Hydraincage för 5000 km. The second time, since a hydraincage cure didn't make much difference, I put some Omega 917 oil additive in the LHM. That fixed it for as long as I had the car on the road. Which means 530 000 km.
If this doesn't help then you've got the good old FDV which can cause all sorts of steering problems as well.
//NiSk