Cracked drainage piping and a fluctuating water table ?????
you should have it camera'd
is there a consistancy to it at all ?? i.e. does it block after a given amount of flow from one or more fixtures? you could attempt to replicate it a few times and see, this would give better insight as to the nature of the problem. The homeowner often knows exactlly whats wrong but doesnt realize it, or have the means to solve it... thats what makes plumbing sometimes a detective type job.