[WS:] I agree with your position on 'within-class" tensions and conflicts, but missing from this discussion is the role of organic intellectuals in manufacturing class unity. This pertains to all classes and socio-political groupings, not just capitalists.
People are not members of the same class in the way they members of the same family or an organization, which have a myriad of social rites that create membership. Instead, class membership arises from perceived similarities and differences e.g. AB have more in common with each other than with C or D, therefore A and B form the same class. Therefore, the creation of a class is an intellectual act, and as such it is typically performed by professional intellectuals. Lenin and Gramsci noted that relation between class and intellectuals and coined terms for them (vanguard party and organic intellectuals.) The connection between different types of intellectuals and class interest was explored by Bourdieu (_Homo Academicus_) and and various students of EE communist (cf. Konrad & Szelenyi, _Intellectuals on the Road to Class Power_)
Based on this, it is reasonable to argue that capitalist class cohesion and unity is primarily maintained not by the members of the class themselves (e.g. businessmen and capitalists) but by their organic intellectuals - academics, esp. economist, jurists, political scientists, (the disciplines that, as Bourdieu argued, are closely linked to conservative political interests) as well as journalists, clergy, artists, film makers etc.
It furthermore follows that capitalists (or working class for that matter) have no concept of what their "interests" are of their own (except for very basic notions of being successful, profitable, etc.) - but rather use the notions of class interests manufactured for them by their organic intellectuals (which is what essentially Lenin argues in _What is to be done_). This may explain why capitalists in countries like the UK, Sweden or Germany accepted the notion of welfare state as being in "their class interests" (it reduced labor militancy and spread costs across the entire society without singling out individual capitalists) - because at that time that notion was widely accepted by experts, including those that were "organic intellectuals" of the capitalist class. This started to unravel only when the consensus among "organic intellectuals" of the capitalist class moved away from managerialism and keynesianism back toward free market.
This of course does not imply that capitalists are dupes swallowing whatever their organic intellectuals dish them out. The relationship is far more complex and dynamic - organic intellectuals certainly respond to demand of individual members of the class they serve, and their views are affected by a larger context and world events, such as the fall of communism, which was widely interpreted as evidence of limitations of managerialism and economic planning.
But while organic intellectuals are responsive to the pressures coming from the class they serve as well from the broader political-economic context, they still play a crucial role in defining and maintaining the notion of "interests" of their respective class and the specific contents that this notion entails. Ignoring their role and focusing instead on class interests as if they naturally existed is a fundamental intellectual error in my opinion. Interests do not just exist, they are manufactured and maintained and it thus makes perfect sense to examine who manufactures them, under what conditions, how, and to what end.
Wojtek