I've had the same issues with my HP Compaq 8200 Elite Small Form Factor.
First I blamed YouTube because the freezes seemed to happen when I would pause a video and the computer would enter sleep mode. Then, I noticed that it still happened (or didn't happen), regardless of what is running.
Today, for example, I woke it up from sleep to see the clock frozen at 8:26 PM from yesterday. It wasn't being used. It had gone to sleep with no programs running on the desktop. The mouse moves, but the desktop is otherwise unresponsive.
Like always, I used the keyboard (Ctrl-Alt-F_) to open a TTY, login, and reboot.
Works fine while it's being actively used (in this case as a HTPC) but completely unreliable for anything critical. It's definitely a sleep issue because I have the same problem with another Lenovo SFF which is flawless so long as it never sleeps, but randomly has the same desktop lockups when sleep is allowed.
Both formerly ran Windows just fine.
Anyway, because they're being used as media players it's just an annoyance, but it does prevent me from taking Solus seriously as a daily driver for my work machines.