Scenario: Customer complains about "internet running slow," however it happens so sporadically and it's never able to be reproduced when the customer calls to complain about it.

Assuming you had you had a public bandwidth test server, could you theoretically use a Mirkrotik script to run a bandwidth test at regular intervals (every 10 minutes?) and then write the results to a file to reference later to see if the router/switch/whatever is actually the problem (or if it's more likely a problem with an application or client device)?