I am having this exact same problem. The 34436 was installed in May of 1997. When the lever is turned all the way to the left the tub spout leaks. My assesment was the same, I concluded it was the stop valves. I tried to find the right sized o-rings, but to no avail.
I ended up calling Grohe parts (630) 582-7711 and asked for technical assistance. They agreed that it was the stop valves (part no. 08.355) and I asked if I could replace just plunger in the stop valves. They said no, but there is a washer set that I can use to replace all of those washers and o-rings. THe part number for that is 47.045, and it lists for $26.00. It has the o-rings for the hot and cold sides and the replacement screens for the stop valves.
I am curretly looking for local distributors so I can buy one on the way home from work.
Per the Grohe technician's advice I plan to replace the rings on the plunger and lube them. I may replace all of the o-rings that are available in the set. I have seen no evidence of pitting or corrosion in the valves, so I am thinking it is just the rings.
So far Grohe's assistance has been very helpful for me.
I do find it kind of strange that a several people have written here with pretty much the same problem in the past couple of months.
I will write back with my results.