Hi inertia

No idea how much of this you have inherited but the structure of the site itself is an indexing quagmire. Available under both http:// and http://www and indexed under both. The lowest common denominator is /residential/home.asp although much of the content is in the commercial folder, referenced to and linked in the navigation as /commercial/home.asp. There are core navigation links that instigate 301s e.g. /blog rather than /blog/ Various pages carry overlapping content, have a look what happens with a search on a piece of text from one of the home pages:

Heat Pumps are a tremendous way to heat your home and provide domestic hot water in a cost effective and sustainable way - Google Search

The page isn't returned, two others are, including the root, which is unreferenced in the site itself, not surprising the pages are in non typical order. Could go on but I'm sure you must be aware of much of this, the site architecture is a real problem.

Your client has added to this by putting at least two nearly duplicate sites (may be more) on the same server, targeting the same market, he is an idiot. The indexed pages from these sites do not want removing, they want page by page 301 redirects put in place. Apart from the weakness, you will appreciate that what is happening is against Google guidelines. They do essentially just filter duplicates out, with no guarantee which will be returned but having these on the same/close IPs could cause a reaction a little stronger than filtering, the 301s really do need putting in place yesterday.

Having eliminated the duplicate text off site, a little rewriting to expand and remove duplication from similar pages will also help, as well as the 301 from non www to www (guessing that's the way you would want to go) Then's a good time to think where the structure and internal linking can be improved, perhaps whether the whole configuration wouldn't be better with at least the residential folder moved to the root and home.asp 301 redirected to the root, rather than the current situation of the root being redirected to residential/home.asp. 301s naturally in place for any changed URLs.

To me, the variable returns you are seeing are just a reflection of inherent weakness, no more at this stage. To turn that round will require your client to be both cooperative and patient, I wish you luck with that and with the site.

By the way, the Google sandbox is a test facility used by their engineers, mainly for new algorithm developments, before they are released into the wild. That's the only one they have.