Should you run into the same problem we had with ActiveCollabs frequently cron that it runs infinitely and eating up all resoruces of your server, mind the following help:
The reason for that problem was in our case that there were just too many (~ 500) mails in the mail inbox which AC observed. This lead to the problem that new mails were no more checked by AC and the cron did never end.
To solve it you should manually log into that mail box and delete all mails.
Afterwards the cron still didn't work for us. The final fix was to uncheck and re-check the mail account ("Active") in the AC admin section.
Afterwards the cron ran correctly and the mails were handled as before the crash.
Please leave a comment, if this was helpful for you.