sporadic "access denied"/"no db selected" errors executing simple query...

Greetings, I have a problem which shows up *intermittently* when executing a simple query. Basically, mysql_query() returns no resource *some* of the time. When I trap the error using mysql_error(), I get the following:

        "No Database Selected"

For kicks, I altered the query to use the fully qualified table name (<db>.<table>), and the error changed to:

        "Access denied for user: 'username@%' to database 'db'"

If I continue after the error, this is followed by:

        "PHP Warning: mysql_num_rows(): supplied argument is not a
         valid MySQL result resource in /home/www/..."

I was able to replicate this on a browser as well as using "ab" with no concurrency. The system will sometimes handle 1000 or more queries without any problems, and then a bunch of errors will appear. I cannot roll this out into a production environment until this is fixed. I would really appreciate any advice. Here's the relevant code:

    $conn = mysql_connect(CHAR_DB,CHAR_DB_USER,CHAR_DB_PASS) or die("Cannot connect to db.");

    mysql_select_db(USER_DB, $conn);

    $sql = "SELECT cash FROM cash WHERE id = '$id'";

    $res = mysql_query($sql);

    if (!$res) {
        error_log('Invalid query: ' . mysql_error());
    }

    $numrows = mysql_num_rows($res);

Thanks!