Debugging and Fixing an OTT App Server

Our application running on Java and MongoDB has three server instances running. One of the three goes into interruptible sleep/Waiting for event to complete (man page process state code S+) from Running (man page process state code R+). This switch is affecting the speed and accessibility of the app, making it stop for multiple hours on end. We want someone to determine the cause and fix this issue or debug the server to determine any other problems if this doesn't affect it.