I think the answer to this question is film age, but I will ask nonetheless.
I bought some Fuji Pro 800Z several years ago for the first time and got some pretty good results with it and bought some more. For various reasons though, I didn't use it much and suffice to say some rolls I bought back in 2010 or thereabouts I use from time to time when light is low. The film has always been in the fridge but its expiry date is March 2012. So it'sa year out. That said, I used one of the rolls back in 2010 and had the same problem I am aout to tell you.
Basically, 800Z is a fast film, so, naturally I use it when light is low or flash not generally acceptable. However, what I have found every time is that any shots taken without flash in even fairly low light or dark light produces stupidly grainy prints and scans. Yet, if I use the film in "normal" light where you can get 1\60th of a sec at f8, the prints look OK.
Now, my question is, if 800Z was a famed grainy film like Delta 3200 (which 800Z is not famed for being), surely all prints from a roll would be grainy - not just the ones where the light was very dark?
So how come the shots where I use it in light where its speed is not actually needed and I could have used 400H or 160S are acceptable, whereas the occasions where I could only use 800Z and the other films would fail produce such grainy unusable results? I have attached two shots shot in the early morning, indoors, in a caravan (sunlight coming in via opposite window) and another taken at midday in the same caravan. As you can see, the two low light shots are very grainy compared to the one shot a few hours later. The prints are obviously not AS grainy, but they are comparatively the same - the two low lights very grainy compraed to the brighter light.
Shot using Nikon F5, 35mm at box speed of ISO800
I only ask because TBH, I will probably get rid of my 3 remaining rolls - it lets me down like this every time, or I am using it wrong.