> Kodak D-76 is notorious for gaining activity(posted 8512 days ago)I wonder how valid that is for packaged D-76; it contains a buffer to prevent that problem, and is said to be actually D-76d, which is consistent with the info in the MSDS.
As for increased graininess, higher development temperatures yield increased graininess (Henry, _Control in Black and White Photography_) with the "best" temperature being 68F-72F.
While I doubt developer age is the problem, four months is a fairly long time to have a jug of D-76 kicking around. I'd suggest simply mixing smaller quantities. If the reason for mixing a gallon is cost of the quart size, you might consider mixing your own developers. A relatively inexpensive balance scale makes it easy; otherwise conversion tables to use tablespoon'n'teaspoon methods can be found.
The simplest is D-23.
D-23:
Water 750ml 125F
Metol 7.5g
Sodium sulfite 100g
Water to make 1.0L
Use straight or diluted up to 1:3. Developing times will be longer than D-76 but neg characteristics will be pretty much the same. Contrary to common opinion, D-23 isn't a "soft-working" developer; it can yield the same curve shape and CI as D-76 if the development time is long enough. I believe most who consider D-23 to be a low contrast developer simply aren't developing long enough.
A D-76 variant, D-76H, is easy to make. It works the same as D-76 but hydroquinone is eliminated to avoid any concerns about the pH increase with age activating hydroquinone.
D-76H
Water 750ml 125F
Metol 2.5g
Sodium sulfite 100g
Borax (20 Mule Team) 2g
Water to make 1.0L