Off notes in hand-tempered chocolate.

Im making some chocolate in a small santha grinder using cocoa powder and butter. Grinding for about 12 hours. I know this is a bit wrong but I wanted to use an alkalised cocoa so I could make it and store it later for baking and using it to make drinking chocolate out of. -(I do not think alkalised cocoa is available in liquor) Unfortunately it is producing an off flavour coming I think from the fat. It hits the top of the pallet! It is not evident until about a week or two after it is made which is when the chocolate should develop a better flavour not worse. Its a 70% dark with quite a high fat content of 35% total fat in the chocolate. (I use 29% cocoa butter and 41% cocoa powder that has a with 22% fat content) I have tried different varieties of and origin of cocoa but it makes no difference. I am wondering if it is in the tempering process as I do temper by hand. If anyone could offer any assistance I would be exceptionally grateful.