Things really do look less saturated in color and less contrasty on a cloudy day. Still, 400VC retains much of its vibrancy, although the low contrast of the light changes the look a quite a bit. Grain won't be affected by the lighting, although it may be a bit more apparent at low contrast. 400VC has fine, but quite noticeable grain, especially with big blowups. Many commercial labs are not careful about replenishment and replacement of their chemicals, so exhausted bleach is a definite possibility. The same problem can cause inconsistency in your results. Another possible problem is that the lab is using Fuji or a third party manufacturer for its processing chemicals. I don't know if this would adversely affect the Kodak film (the Fuji and Kodak processes seem to be mostly compatible), but it might.