Couple of questions before I can try to even guess.
What does the film look like? Is it overly thin or overly dense in the image area?
What does the film rebate (outside the image area) look like? I know the numbers are there but is the rebate clear or if you look closely, can you see fairly even coat of what looks like an exposure to light?
You said your D-76 is older but how was it stored? In full bottle or less than full bottle?