Hi Andy,
I still do not understand your test procedures enough to offer good advice. Can you answer the following questions:
1. When you say an "ESD Test house" tested the IC against the MIL standard to 8kV, do you know what equipment they used and how they performed the test?
2. For your system level test you are doing yourself, what is your "ESD Equipment"?
3. Can you describe the system a little, you mention your ananlog IC, and a "digital" part. What is the digital part and how is it related to your analog IC, what is their relation to the rest of the system. (This question may be far too broad for this forum as I would almost need a schematic with power/GND routing descriptions.)
I will say this, the ESD testing of an IC has very little correlation to system level testing. For an example, please read my comment in this post:
http://www.designers-guide.org/Forum/YaBB.pl?num=1226827296So even if your Part is tested to the MIL standard, it will have no bearing on how that part or a system using that part will pass the MIL standard. The primary reason is that when you test the IC, you strictly control the return current paths, but in a system, the return current paths are dictated by the system design and may not be the same as the IC test, therefore the test results may not be the same.
Essentially, System tests do not correlate well with standalone IC tests and vice versa. System test have also been notorious in that reliable and repeatable system models have yet to be developed in the industry.
Its particularly bad when people try testing with "ESD zap guns". They are cheap and available but they do not make good ESD testers, either for an IC or a system. They only thing they are really useful for is testing conductance and discharged requirements for standards such as MIL, CE mark, UL, etc.
Hope this helps.
-Stephen