where is the problem?

Past night during a recording session i face for the first time the following problems.
First i saw a longer time to save the project files: 6 mb, normally Cubase takes 2 seconds, but yesterday tooks 2-3 minutes every time i saved.
Then during recording Cubase hangs, remaining frozen for 1-2 minutes then Cubase come back to normality but with a broken record part.

I felt a disk problem so i made a check disk but all tests, including SMART data, were 100% reliable on all disks
Then i made a full backup and reboot the computer. After that Cubase was ok, fast saving and no problems on recording, but was too late so i tested only for 10 minutes.
My computer is a pc build 2 years ago, I-7 24 gb ram, win 7 and cubase 9, RME Multiface.

I repeat, i never had similar issue before and your suggestions are welcome

Sorry to get back to this problem again, but this really makes me crazy.
Every session i made in last two months suffered from the same unknow problem with unexpected delays when saving song or muting audio of some tracks/part during play.
It can happens only 1-2 times then it’s ok for hours, sometime more often.
The pc it’s only for music so no internet/office or other software on it, so i can tested only with cubase. During backup it seems to work flawlessly.
Process analysis during the saving problem shows that cubase process is frozen then after many seconds restart then is frozen again and so on…
The system+programs are on a ssd, samples and songs in a 2tb hd.
Thanks for any help suggestion about it.

It sounds too simple, but is the disc drive anywhere near to full or badly fragmented? Can you try copying the project to an external USB drive and test some saves and renders to see if you can reproduce the fault on that other drive? I have also had a problem with dodgy SATA cables once or twice, seeming to cause a freeze up, worth checking all connections.

Is your power supply browning out perhaps? Or system running too hot? Either will cause all sorts of symptoms, throttling CPU, starving power to drives etc.

It’s probably a hardware issue, but alternatively have you started using a particular plug in at the same time the fault started? For example I had a problem with BFD2 causing system to freeze on Cubase exit. In fairness FXPansion rapidly updated their software and cured it, but it was weird for a bit until I isolated the cause. Projects that didn’t use that plug worked perfectly.