There are two things to do before we can definitively state the cause;
1. Upgrade the Netprobe: The version you are running may be old and there have been too many changes to list including fixes to a variety of plugins. Upgrade to the latest release.
2. Enable memory debug to the Netprobe log. This can help narrow down which plugin and at what time consumes memory. These can be legitimate and are not always leaks. To enable this debug:
a. On the specific Probe config in the Gateway go to the Debug tab
b. Click on the first/top "Add New" row and then in the two new lines added enter "SAMPLING" and "SAMPLINGMEM" (both no quotes, all caps) in the respective Module fields, leaving all other fields untouched.
c. Save the config and observe the new liines in the Netprobe log showing the start and end of each sample plus the delta of memory consumption.
Once you have done this and the problem still occurs with the upgraded probe, please send us the log file produced (firstname.lastname@example.org) .