Kodak D-76 is notorious for gaining activity as it ages. The pH gradually declines (I think) with time and that activates the hydroquinone (which is normally inactive). See Anchell's The Film Development Cookbook for more.(posted 8513 days ago)What is happening, as a result: your film is being overdeveloped. This increases grain.
The solution: change developers, or use fresher D76.
I used to use Ilford ID-11 because you could buy 600 mL packages. I don't think you can any longer, but they still have 1 L packages. ID- 11 is functionally identical to D-76, and, in fact, some believe more chemically pure to the original D-76 than the current Kodak product.