yeah it happens.actually both of them use different parameters to decide upon the relevancy of the website. e.g. Google discounts the nofollow links but Yahoo! counts them for the ranking purposes. if your website is ranking well on Google then you should not worry about Yahoo! rankings as most of the traffic is generated from Google

The problem is my site ranks fantastically in Yahoo! but not so well in google. Ranking No1 in Yahoo! for most terms and hovering around 20-30 in google. I do wish it was the other way! Building one way links I thought would do the trick, but still the same old seesaw ride.

How do you think I could combat this to rank high for both (for all)?

Some of the up and down effect that you experience in the Google SERPs is a natural occurence as a results of ongoing crawling, content comparing, web page awarding and refreshing of datacentres. SEOs that track their rankings daily frequently notice a ranking fluctuation period, that could last a week or more, and then are happy to be back in a stabilization period, that could endure a couple of weeks and up to a month. In the stabilization period is where your rankings are most accurate read.

From what you are saying, I'm inclined to believe that it could be coincidental that when you observe a fluctuation period with your Google rankings your Yahoo and MSN ranking peak.

I would think that you are on the right track by simply continuing to offer the search engines an evolving website. I wouldn't be wasting my time trying to empower my content externally using any of the trendy link popularity boosting schemes, nor would I try to create the illusion that my content is important by link relevancy manipulation. As far as seeking awards from links, I'd be focusing on developing a sound internal linking structure, one that emphasizes my most important webpages.