Thanks Sachys, after vacuum cleaning my CPU fan and setting the DEP (c.f. other thread) I now have whooping 13 Frames per second. Thats some improvement, still not getting close to the average 25-30 I had the day before yesterday.
I ordered a 400ml can of compressed air to blow away any dust that settled on the fans, maybe that helps.

Hmmm... I take it you've rebooted, defragged and done a c-clean as well already - I'd also ensure the steam in game overlay is turned off for the game.

Good morning Sachys,
what do you mean by c-clean?
Oh and defragged? Is this seriously an issue with an SSD card?
I had it reinstalled there after it turned out to be lagging unbearbly on my standard ATA HD.

I can't figure out what happend yesterday night. It's not that I installed anything or did anything unusual. It just stopped beeing playable.

C-clean - as in use a program like c-cleaner or the inbuilt thingy with windows to get rid of any "dross" from your drive - temp files and the likes usually.

I've no idea about an SSD - didnt realise thats what you have, but like I said I presume you'd have already done all that I mentioned as general troubleshooting.

As to the game the DEP trick has worked for plenty of people, so theres obviously something else affecting it but I'm sadly at a loss to explain what.

You could always try exclusion method. Try multiple games especially different kinds when it comes to technical capabilities. Also use software like memtest to exclude it's not your RAM, chkdsk for bad sectors on hard drive etc..

Do you run any power saving features? Maybe they're screwed so they kick in even when you're in a game, can happen with dual or CF/SLI setups.

My bet is bad drivers, hardware failure on either GPU or mothercard. Anything else usually give other symptoms.

Just had a thought - there may be a background process or "service" that running SR3 with the lag started up thats causing the continued problem.

It happens as soon as I have people around, so it actually happens inside the building but outside your first "crib" where you are alone.
All of the other games are fine. Dead Island runs with over 100 Frames with a minimum of 65 with all Specs on Maximum.
Skyrim works okay, in High settings with the High-Res mod.
Haven't checked the RAM yet.
Doubt it's the drive as I moved the game from HDD to SSD drive without any improvements after it became unplayable. If its a hardware failure I wonder why only SR3 is affected and why I haven't had any problems before.
I never used powersaving features, so I doubt this has anything to do with it but will have to check this as well.
Don't know how to check all the background processes. Task Manager? I have to look into that too.

Thank you people for your help thus far.

Now i think about another thing , i know DeepSilver like to use Securom.

I have a huge numbers of posts in the Sacred 2 forums , i know " a bit" (lol) Deepsilver -> to the point ; some people suspected the DRM was respeonsible to slow down the game (was one of the factor, there were some others, but still..)

if the game uses the same DRM schemes, it's possible it could one of the cause. But that's just a theory ^^ , but if the game was badly ported as some others said , that's also a big factor in the drop of the fps. :(

It's true that SecuROM affects Sacred 2 stability and performance, that's why it's recommended to use the latest fix for the game so it's possible it could be the same with Saints Row. That's an easy solution so I would recommend trying it at least.