I asked the manufacturer about my problems with my latest bottle of TiteBond III (thinner and lighter color). Here's the response:
"The only change to Titebond III was a qualification of a different raw material supplier for the wood flour we use. If the product is frozen and thawed, the viscosity can decrease but is typically within our specification range for this product. Observations of low viscosity may be due to the cold winter this year and a larger amount of shipments of adhesives seeing freezing temperatures. Be assured that this will in no way affect the strength of the adhesive bonds. The color change is due to use of a lighter/cleaner wood flour additive as we strive to provide a higher quality product. If a darker product is desired we offer Titebond II Dark which can be mixed with the Titebond III to increase the dark color. In addition, water soluble dyes and pigments can be added to change the color as needed."
Seems to be a reasonable explanation but I'll be watching for a return to high viscosity in summer shipments :-).