Known affected version SOAPUI CE 4.5.0, 4.5.1.
Symptoms: Ru load test with multiple threads with little delays between threads over a time interval. Many of the threads eventually stuck in the sleep state at line 86 of 4.5.1 ModelItemIconAnimator.java. So the number of the running threads dwindles to only a few threads. The issue is the poor concurrently in the start() method of ModelItemIconAnimator.java.
Attached is the original code and a fix