My bike is similar to what Ayers describes, Cro-Moly frame mountain bike, looks a lot like a road bike but the tires are 2" wide slicks with a high center instead of the fat knobby tires you find on most MB's or the narrow road tires you find on typical road bikes. The tires are nice in that on pavement there's maybe 1/2" on the asphalt, but when you get off pavement they sink in and you have the full 2" width for traction.

I've never had a problem with stability going fast, but maybe it's because my frame is more like a road bike's.