Some JMeter threads stop processing during a variable time

Hello,

I am using JMeter 2.11.
The following parameters are defined in the jmeter.bat file

set HEAP=-Xms512m -Xmx12144m
set NEW=-XX:NewSize=128m -XX:MaxNewSize=128m
set SURVIVOR=-XX:SurvivorRatio=8 -XX:TargetSurvivorRatio=50%
set TENURING=-XX:MaxTenuringThreshold=2
set RMIGC=-Dsun.rmi.dgc.client.gcInterval=600000 -Dsun.rmi.dgc.server.gcInterval=600000
set PERM=-XX:PermSize=64m -XX:MaxPermSize=64m

We made a scenario that needs while statement.
If we remove the while statement, JMeter manages to handle 50 users concurrently.
If we add the while statement, around 80% of JMeter user threads are executed correctly.
20% of JMeter user threads stop during a variable period : sometimes 15 minutes, sometimes 40 minutes, sometimes an hour.
And then the scenario keeps on with the next statement.
By tracing the activity with debug sampler, it just stops anywhere usually on a timer during 40 minutes for example and after 40 minutes, it sends again HTTP requests (the problem is that my IIS application session is timed out and all requests failed).
It seems that the more we add debug sampler, the more JMeter works correctly.
There is no log...

We tried the following :

- Change JMeter.bat settings

- Upgrade JMeter version

- Increase timers in order the scenario to be less stressful

Nothing works. We still have the problem. I am therefore wondering if JMeter has reached its maximum capacity.
It should be noted that the injector CPU is around 60% and the memory is ok.

If anyone has any idea concerning this problem, I would really appreciate.

Regards

Sylvie