Why do you place the system under a load when you are testing for errors?

You will need a test program (also called a diagnostic) that exercises the functions that are suspected of making errors. You may also have to run this program under a variety of different environmental conditions (e.g. high temperature, low temperature, higher supply voltages, lower supply voltages) to stress the hardware more and increase the chance of finding intermittent errors.

Such testing can often be quite frustrating, I did it on systems that had to operate on full Military temperature range (-54C to +75C) for over a decade and some problems took more than a month of varying test conditions and rerunning tests over and over (even sometimes writing special diagnostics just for that problem) before the cause of the problem could be isolated.