Unless you have made major on-page changes, the drop in rank is likely due to changes in Yahoo's algorithms. Google focuses on offpage elements, with a secondary consideration for on page elements, and MSN does just the opposite, and I am unsure what Yahoo's priorities are based on, but they are focused much more on on-page factors than off.

Sem is right, in that Google is really the only company right now (other than Ask) with a primary focus on search. Yahoo was one of the first directories, then a search engine, and now has branched into more of a social networking / shopping / search site (yes a portal) in large part because Google has taken away a good portion of Yahoo's market share. Microsoft has tried many times to break into search and is still hanging on at third place (by most accounts). However, each of these search engines drives it's own traffic. Google draws most of the searches, but Yahoo and MSN each have their own audiences that can be very valuable as well, especially Yahoo, since their engine powers numerous other sites.

As far as the influence of a hosting company on the results, this is unlikely to have a drastic overnight effect like what you are describing. I haven't seen or heard of any evidence of this (other than hearing third-party reports of hosts being banned for hosting multiple domains that were considered unsavory, but this is typically the exception rather than the rule). I suspect that if there were any evidence (which would be hard to find) it would be a major scandal. As far as I know, though, there aren't any major hosting concerns on the board of directors of any of the big 3. Google is mostly mixed with Apple, Yahoo is most closely tied with their own hosting which is mostly free sites and small business hosting, a relatively small portion of their business and most likely not worth the risks of manipulating the results, and if Microsoft wants to crush a hosting company, I doubt they need to do anything with their search results. Also, if Yahoo or MSN were to do anything like this and get caught, they would lose their market share to Google, and if Google did so, they would lose the entire company which is primarily focused on search.

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