I was re-reading Heraklia's post on the Gracchi and the orders relationship as a cause to the Republic shakiness.
The feelings I get from my readings are that she hits the true issue...
The way I feel - IMHO - is that the problems started with the Equites getting more and more economical power from better exploiting the possibilities given by the provinces, whereas the Senators went on being focused on political power. This brought to a discrepancy in the social tissue of the Republic, because those who drove internal and foreign politics (and took decisions on both subjects) weren't anymore those who had the most gain (or harm) as a consequence of their decisions.
Plus... the whole 'mos maiorum' issue is something I feel really itchy about... it is an utopia that has meant very different things for very different people... something too easy to be exploited as contingent convenience required. Perhaps only the Catons were honest about it (and still I personally have some doubts at Caton the younger)...