lock file of cronjob never removed

Dear,

enviroment:
- multi server enviroment (1 master, 10 and more slave);
- ispconfig 3.0.5.4p3

On some server, happen that the cronjob script (server.sh) run a very long time, randomly (i mean: sometimes took 1 second to run, others time took 20-30 minutes). And this is totally random event (not related to particular job, or others). Could happen also is no job are in the queue of this servers.

I have activated also the debug stuff, but no information are given.
If a kill manually the process, remove the lock file and then lunch the process manually he works correctly.

What could be? Any advise on how can use in order to debug them?

ty