Hmm, yeah. While not common temp sensors can be completely wrong. But the good news is that (from what I've read) they either swing way out of line one way or the other and aren't just a little off. (temps way to high/low to be normal)
What I'd recommend is to first check that it is indeed throttling by running a game and having CPU-Z open somewhere where you can see it and make sure its actually downclocking the CPU when you get the lag spikes.
If so, contact AMD and see what they say.
Oh, one more thing, what are your idle temps? Are they the same in windows as they are in the bios? (more or less the same anyhow, windows adds a bit of load, not much though.)