I also have an '88 M6 and this board has been of great help to me also.
The hydraulic system generates quite a bit of heat during normal operation and relies on fluid flow through the system to cool itself. On some cars, there is a cooling pipe loop behind the bumper to aid in this effort. The only restriction I can think of is a filter in the resevoir. If this filter has never been changed, that could be your problem. Sounds like once your fluid cooled down, things were normal. Hope this helps.

Regards,
Jim Taft
88 M6
I own an '88 M6, but this board has often been of help to me...

After a spirited two hour drive, I entered a small town and slowed for several lights. Suddenly, I heard a loud grinding noise up front and could feel it in the steering wheel. Pulling over, I checked the power steering level and found the reservoir to be full of soapy bubbles. I waited for these to subside and then checked the fluid level. It seemed OK, but the fluid was a coppery/muddy color. So I waited half an hour and checked again. This time I added fluid, but not a lot. Starting the car, all seemed OK, so I carefully started home. Two hours later, I'm home with no more problems! What happened??

I had a similar experience recently the day after a long drive, so I checked for a leak -- finding none, I took it in to a reliable shop and they found no leak, either. Could it be that the car leaks only when driven hard? Plus if the fluid boiled (the soapy bubbles??) is it ruined and in need of replacement??

Any thoughts, ideas etc. most appreciated.

Dave M
'88 M6
'95 525