A X4 955 is not going to choke on any game, period.
That's what I was thinking too, but in at least one of these games, it's definitely pegging one of the CPU cores.
Here are my system specs:
AMD PhenomII X4 955 Black Edition 3.2GHz AM3 CPU
ASUS M4A78T-E 790GX Motherboard
XFX Radeon HD 4870 X2 2GB 512-bit Video Card
OCZ Platinum 4GB DDR3 RAM
Corsair 750W 12V PSU
What I have noticed so far from doing a little troubleshooting is that while playing Rise of Legends, my CPU usage is at about 25% most of the time, as in, it's only capable of using one core and it's running the core at full power ALL the time. On the complete opposite side of the spectrum, when I play WoW, it freezes for a moment in a manner similar to the way Rise of Legends does, but when that happens, the CPU usage drops to zero and the game only starts again when it comes back up. And it's not just that the game's usage drops, it's that the entire CPU seems to stop for a second or so.
Those are the two games that I've been using enough recently to notice the problem and do a little troubleshooting, but I know that World of Goo and Starcraft don't cause any problems, and that TF2 does seems to have a similar issue. I've also looked into the possibility of the problem being caused by the Cool 'n' Quiet feature on my CPU, but I've tried both games with that feature both on and off and the problem still occurs.
Hopefully this info will help in narrowing down the problem