Running a TPC-C Benchmark Without sleep() i.e. key-in + think time
Apologies for the late response to this, but there is a key reason as to why you are not observing an expected smooth behavior:  You have a client-server model violation when you eliminate think time.

The client server model is built upon a foundation that not all user are using system resources at the same time.  This is different than the mainframe model with terminals where all users are being serviced at the same time by the core host.   With client-server distributed processing exists and there is a delay between requests during which the resources are free'd to service the needs of other inbound users.  Normally when you place a host under load with think time these free internals tend to become smaller and smaller and as load increases you eventually reach a tipping point of no free resources to service requests.    

By eliminating the think time you eliminate the free resource window to service additional requests and your tests become invalid as a predictor in real world performance.   If you ever hear a performance tester tell you that they have eliminated think time and every one user is equal to n real world users, fire them on the spot.