Update: I have also tried staying under 2 requests per second (at 10,000 requests/sec the rate should be 2.77req/sec), but no luck. My requests are actually not returning any results now, so it's not strictly a 999, but a <result/>.
<query xmlns:yahoo="http://www.yahooapis.com/v1/base.rng" yahoo:count="0" yahoo:created="2012-07-17T10:22:45Z" yahoo:lang="en-US"><diagnostics><publiclyCallable>false</publiclyCallable><user-time>141</user-time><service-time>139</service-time><build-version>28958</build-version></diagnostics><results/></query><!-- total: 141 -->
<!-- engine1.yql.sp2.yahoo.com -->
Your help is appreciated!
JJ