I suppose you have more than 4*275 clients.
After yoiu reach InUseConnectionCount=275 on one instance the next calls to a connection will wait the 'BlockingTimeout' of the DS configuration.
This will often end up in such situations, many threads are waiting for a connection and block memory and resources, the GC can not free memory, the server gets slower and you will have a crash because of too much GC load.
You might
- increase the number of connections
might not help because you must handle the number of threads in one instance (maybe increase memory)
- add JBoss instances
- limit the number of web sessions