Well, I purchased The Grohe replacement washer set (part number 47.045) from a local Grohe dealer. I replaced all three o-rings on the plunger in the stop valve on both sides. I also replaced an o-ring in the outside of the stop valve (the part that holds the screen) on the hot and cold sides. The leaking stopped.
The replacement washer set contains far more washers than I needed for this job, but that is OK. I hope I never need the remaining washers to replace o-rings in the thermoelement.
I was happy to replace the middle washer on the valve stop plunger and the washer by the screen, as both were pretty well flattened, and one fell apart as I tried to remove it.
The washer replacement kit came with a small amount of Grohe's lubricant, which I applied before re-assembling the stop valve, Things went back together pretty easily.
SO, overall Grohe support and thier local dealer helped me get a solution to this problem. I think there is another washer set available that replaces just the two washers on the stop valve assembly (near the screen). This washer set may have done the trick as well. I do not have that part number in front of me, but it consists of a total of 4 washers, two for each side.
This is a great forum, and I will probably return seeking advice on other stuff in the future.