i have a 3770k and a gtx 770, I can run ultra minus ubersampling at 70-80 fps.

but after about 10 minutes it drops to 25 to 30 fps and i have to restart my computer.

This is the only game that does this.

Edit

Once this has happened even low settings only gives me about 35 fps

Sorry I can't help you, but I started to have similar issues. In addition to occasional FPS drops (fairly noticeable, as GoG has none FPS meter, I used Steam version to check, values are around 20 or lower) and frequent micro-stutter (seems to be permanent/regular event really), my version of Witcher 2 started to have even weirder glitches: for example, game lost all sounds, excluding thosemade by Geralt himself. I was surprised to hear his clothes made noise when he reaches for his medallion, which is cool, but lack of other sounds (ambient, monsters, music) was rather strange. After reboot sound returned, but game no longer shows enemies' "nametag", making it impossible to "lock" on them and see riposte suggestion icon or aim a spell for distant shot, for example. I really don't know what to do, I tried to reinstall, to verify integrity cache, nothing is helping. But I remember game been working fine before.

I guess you checked for latest drivers. Cause of the timespan, I would further investigate about temperatures (CPU, GPU).