Did you use 1 quart pack or 1 gallon pack?(posted 8115 days ago)If you used a gallon pack, start with about 3 to 3.5 liters of water, dissolve everything, and adjust the final volume to 3.85 liter.
I don't know how hard it is to dissolve all powder of a gallon package into 800ml of water, but if everything clears up in a day or two, it should be fine. If you want to make sure, use a small piece of film (edges that are cut off from 35mm strips would be fine) to check if the developer is alive.
Stock D-76 should be a clear solution free of visible precipitation. At room temperature, whatever resisting to dissolve is probably metol or hydroquinone. Don't filter them out. Let it sit and dissolve.
If you can't stand this painful dissolving process, mix D-76 from scratch or use Ilford ID-11.