Yesterday we made an update to our McAfee anti virus.
We have many services. One Map Service have a maximum instances of 20 but it is never getting more then 8 instances running.
After the update it got 20 instances running (on the two servers we have). Other map services and image services was not affected.
The number of requests was not too big and publishing a different map service on the same data worked fine.
When we stopped the anti virus everything went back to normal.