Engine issue. The game heavily relies only on one CPU core. Your rig doesn't matter much only if it isn't some 20 year old laptop.

Turn off V-Sync.

Turn Settings low if needed.

Inside the game:
= = =
Outside the game:

Overclock the Cpu which may not work.

Turn off any game enhancing apps the device might be using (whether it be Nvidia, Intel or something else) which is getting more common these days including the troublesome Nahemic sound enhancing app which makes older games run with issues.

Turn off Anti Aliasing.

I've purchased Torchlight 2 on GOG and had the same problem.
Running a powerful machine with an RTX 4090.
I am using a frame limiter at 122fps and it is usually capped at that value, however often experiencing severe frame drops down to 50-30 and sometimes below and it stays there resolving back to the 122fps cap after a while, repeating this behaviour randomly.

Been testing around and found out the culprit was Anti Aliasing. Turning it off resolved the random low frame rates during my play sessions.
I think this game uses an out of date AA preset which messes up in combination of no support to utilize multiple cores. I've everything else set to the highest settings and only turned off AA (and VSync, which was off by default). No problems ever since, game runs butter smooth now.

Hope this helps to others who might come along this thread.

Cheers
TalentX

I have a gaming rig, but I've capped the fps on a bunch of my older games because they just play much better at that fps rate. A lot of older games were designed to run best at 30 fps because that is all computer could do back then. The game doesn't look any different for me, but I'll admit that my monitor is an older one, so maybe that's why it still looks great?

Anyway, I had a lot of problems with this game overheating my machine, which didn't make sense on a gaming computer that should easily handle it. After 2 years of only being able to play during the coolest months of the year, I did a ton of research and finally found this answer and it worked!