Old thread, but I am wondering if this was ever resolved. I have a similar issue.
http://www.peachparts.com/shopforum/tech-help/334783-w126-sudden-heat-loss.html
In short I have no heat at higher RPM, including sitting in the driveway. I have no viscus fan, so there is no air movement when sitting still. I just replaced the t-stat, which was pretty new, and there has been no change.
My last two thoughts are blown head gasket with exhaust leaking into coolant or the water pump. Although I would think with the water pump going bad the engine would run hotter, not cooler.