IIS 10 random 500 errors - possibly from url rewrite

Question

This is a production IIS web server that has been up and running fine for a month. There are 6 sites on the server, and last week one site started to give random 500 errors. The was one of two site using URL Rewrite. We spent about 2 1/2 hours with a well versed consultant who could not give a definitive answer. The rewrite rule was:

<httpErrors existingResponse="PassThrough" />
<rewrite>
  <rules>
    <rule name="Redirect .cfm extension" stopProcessing="false">
      <match url="^(.*).cfm$" ignoreCase="true" />
    <conditions logicalGrouping="MatchAny">
      <add input="{URL}" pattern="(.*).cfm$" ignoreCase="false" />
    </conditions>
      <action type="Redirect" url="{R:1}" redirectType="Permanent" />
    </rule>
    <rule name="hide .cfm extension" stopProcessing="true">
      <match url="^(.*)$" ignoreCase="true" />
    <conditions>
      <add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
      <add input="{REQUEST_FILENAME}" matchType="IsDirectory" negate="true" />
      <add input="{REQUEST_FILENAME}.cfm" matchType="IsFile" />
    </conditions>
      <action type="Rewrite" url="{R:0}.cfm" />
    </rule>
</rules>

The failed trace showed

We removed that rule have have been trouble free since.

Now the other site using URL Rewrite is giving random errors, but this one is a bit more complicated to fix as it's or API.

<httpErrors existingResponse="PassThrough" />
    <rewrite>
        <rules>
            <rule name="ApiWithControllerAndId" stopProcessing="true">
                <match url="^([^/]+)/([^/]+)/([^/]+)/?$" />
                <conditions>
                    <add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
                    <add input="{REQUEST_FILENAME}" matchType="IsDirectory" negate="true" />
                </conditions>
                <action type="Rewrite" url="index.cfm?controller={R:1}&amp;method={R:2}&amp;id={R:3}" />
            </rule>
            <rule name="ApiWithController" stopProcessing="true">
                <match url="^([^/]+)/([^/]+)/?$" />
                <conditions>
                    <add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
                    <add input="{REQUEST_FILENAME}" matchType="IsDirectory" negate="true" />
                </conditions>
                <action type="Rewrite" url="index.cfm?controller={R:1}&amp;method={R:2}" />
            </rule>
        </rules>
    </rewrite>

They come and go seemingly at random. There's nothing we've hit upon that can guarantee a 500 response. We have an identical staging server that has not given any 500 errors. I just enabled failed request tracing on this site but so far have no captured a 500 error.