So many things could be wrong here.
1) Malware/Virus/Worm. If you think your computer is immune to them, that is your first mistake. They can fire off in the background and bring a computer to its knees. There is no such thing as a computer being immune to these programs. If you have no taken the proper time to configure the security of your system, and practice safe browsing habits, your computer is open to the world no matter what anti-xxxx software you use.
2) The 9100 is a very, very, very low end video device requiring the CPU to do most of the work. If the CPU gets busy doing other things, for any period of time at all, the frame rate will suffer accordingly. This could be a DoS attack from the Internet, running P2P software (many other games use P2P software for patches and updates and will run in the background when the game is not active. WoW is one of the more popular games doing that.)
3) If this is a laptop, and the system gets hot, the system will slow things down to reduce power consumption until the system is cooled off.
Those are but some of the highlights which could cause what you describe.