Not to imply that this is the definitive cause but just another facet to consider along with contamination.... I found this in a 1993 George Fix article on Diacetyl, Which turned out to be the problem I was having. My diacetyl problems would show up in the bottle but after cold conditioning for a few weeks, would mostly disappear.
"The reaction acetolactic acid -> diacetyl is of the redox type (2). Acetolactic acid is oxidized to diacetyl, and other constituents (for example, various aldehydes as well as wort-derived melanoidins and tannins) are reduced. In all of the mechanisms described so far in this article, this is done enzymatically by microbes, culture yeast, and, in adverse cases, by other guests in our worts. The reaction can occur nonenzymatically, however, in the presence of an appropriate oxidizing agent. Indeed, a widely observed but little discussed phenomenon occurs when diacetyl appears spontaneously in a beer that seemed to have normal flavors. Strong evidence indicates that this can occur when marginally dysfunctional yeast have been used in the main fermentation -- they tend not to metabolize all the acetolactic acid in the wort. The acetolactic acid spills over into the finished beer and later is oxidized to diacetyl. Mechanical abuse of packaged beer can promote this; headspace air is the oxidizing agent. Elevated temperatures augment the effect. I have seen cases in which wort constituents (melanoidins and tannins), oxidized on the hot side in wort production, were passed on to the final beer, only to play the role of oxidizer there."