JMeter - Performance slows down after several logins

We are running performance tests, using jmeter. The jmeter script does this: Login Call a microflow (via a deeplink) logout   The jmeter, start many threads that do many loops (ex 20 threads and each thread doing 20 iterations of the 3 steps above). Each loop within a thread takes a userid from a csv file containing 10 userids (aebusdevama,b,c,d,e,f,..)   The tests run fine, EXCEPT the login. The login exhibits this strange behavior: The logins start OK (login takes a few secs), then increases, then goes down, then goes up even higher, then down again ex : 5secs, 1min, 5secs, 2 mins, ... etc until 12mins or higher ! Login xas : {"action":"login","params":{"username":"${userid}","password":"${password}"}}
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