It’s always a good idea to keep building back links. Search engines prefer to find you, and the way they do it is by crawling sites through hypertext links. The more text links you have the better chance you have of getting crawled more often by SE’s crawlers. The higher PR you have the more you are found by Google etc. MSN has the most active bot at this time, it is constantly out looking for fresh content to update their index. They update often, a lot more than the others. Right now MSN is doing some strange things compared to their patterns in the past. There is a lot of speculation right now regarding MSN, as many are analyzing to see what happens next. They are doing things differently that is for sure. They started using DMOZ descriptions in the SERP’s and they have changed their algorithm.
If your site was newer I would tell you that it is probably just “in the mix” somewhere in the indexing cycle. Sometimes newer sites get tossed in and out for awhile before they are indexed fully.
If your linking partners are not questionable than it may be related to the IP change when you changed hosting companies from one country to another.
Changing your META tags would mean a little shuffle, but your site should still be in the index showing when you query for the url. You should just get re-listed under the new keywords if that were the case.
I would go to other SEO forums and ask for feedback regarding your MSN dilemma. If you can find forums that will review your site, try to ask for recommendations from members, and ask specifically about what MSN may be doing to it. This one has got me stumped and I do not have sufficient enough time to analyze the whole thing for you, but others might.
I am not sure exactly what effects changing IP’s has, so I cannot give you a solid answer regarding that question, but others here might have more knowledge, or possibly in another SEO forum.
Good luck, be sure to update us here when you find your solution, so we can learn along with you.