Ubuntu and Lubuntu 16.04.1 freezing

Hello there, I work in a lab with 13 PCs, 12 of them running Lubuntu and the last one running Ubuntu. Recently I dicided to update these systems so I did update 2 of them (the Ubuntu and one of the Lubuntu, I've used apt-get update and apt-get dist-upgrade). Since then the Lubuntu and the Ubuntu that I have updated are freezing randomly (the PC freezes and I have to restart it using the power button of the case), I can't find any particular reason that would lead these PCs to freeze, they have 4GBs of RAM (Lubuntu) and 8GBs of RAM (Ubuntu) and run all the time with more than 2,5GB free (the Lubuntu run most of times with over than 80% of CPU utilization, though). These two PCs with problems run Ubuntu 16.04.1 and Kernel 4.4.0-34, the others that run without any problem are using Ubuntu 16.04 and Kernel 4.4.0-24

I would really appreciate if you could help me understand and fix what's causing this problem, if I missed or if you need any extra information just tell me and I'll post it.