This sounds like the classic shared hosting conundrum. Most shared hosting servers are wide open to crossover attacks, where a back-door or cron task on one site will infect many or all of the other sites on the server. If you can’t find the root source of the threat on any of your sites then it could be coming from a site on another account and there may not be much you can do about that.
I suggest switching to another, more secure, hosting environment. I do offer Super Secure Hosting for just such a problem as this and I can guarantee that your sites will not get re-infected on any of my servers. If you are interested in switching to my Super Secure Hosting then you can email me directly and we can discuss you particular hosting needs. If you are going to move to any other hosting providers, I suggest that you spread out your sites on different accounts/servers to minimize the crossover threat and isolate any problems you may bring over to the new server (if you put an infected site on any of my servers it would not be a problem).