What is the best approach for fixing intermittent bugs in Access?

[Minty]

You make a good point, but if you scroll way back to my OP, you will see that we use a temporary wired Ethernet network. I would never have trusted wifi for connections to a back end database..

Apologies - I missed that.
It might be worth running some pings to the SQL server - nice low latency connections with no drops are normally rock solid in my experience.

[Micron]

Maybe I missed this but where is it explained that 20 concurrent users are not using the same fe? Might be something that you would never do but it's not obvious here.

Very difficult to find such a problem cause when the testing and production environments are not the same. I think the best you can do is break on all pc's, otherwise it's hit and miss and failure is likely to occur on the ones you don't pick. You might even find that different operations are at play by doing so. Unless you can set up a test before crunch time I'd say don't limit your ability to uncover the problem. Users would understand you're trying to solve a problem if you tell them up front that there are going to be interruptions.