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I'd forget about AltaVista anyway - are they still around? :)
Seriously: What's your site's strengths, SEO-wise? Lots of links? Lots of content? Something else? That'd give a clue as to why you're doing well in one search engine and not in the others.
The other possibility is that Google's latest algo update has bounced you up in the rankings. If that's the case it's just a matter of timing.