What CPU is it a NewCastle (512KB L2) or a ClawHammer (1MB L2)?
If its a NC, then your multiplier is x12, and 221x12 is 2.65GHz which at 1.55v is a very impressive overclock. I'm pretty sure you're at your chip's limit. If you want to get a higher HTT speed, lower the multiplier and increase the HTT...