For about a month I was considered a "top contributor" then, that designation disappeared from my avatar?

You only have a TC badge for a week. It is awarded on Monday morning. It stays with you for a week. If you haven't answered and received enough best answers in the category during that week, the badge does not get renewed on the following Monday morning.

It is the same for every TC, don't answer and get enough best answers in the category, and the badge disappears. You have to maintain your level of activity in the category to retain the Top Contributor badge.

Becoming a TC has to do with the amount of "best answers" received in a particular category. Rumor has it that the best answer percentage has to be at least 10% in a particular category in the last 4-6 weeks. And during that time period you must have answered at least 45 questions. You can get the TC badge for up to 3 categories, mouseover any users avatar and you'll find out in which categories they are most knowledgeable. You may loose this badge again when you get less active or receive less "best answers".

The same thing happened to me. It's because I stopped keeping up with all the other top contributors. I got it back eventually just by starting to answer a lot of questions again.

Someone must have iced you man. I'm pretty sure they determine top contributors by # of answers/best answer ratio. With a minimum of a certain amount of total answers to even be considered. I would recommend shooting for more best answers I suppose.

I can't be sure as ive never been there but i think you have to keep up the pace of asking and answering to keep hold of it.

they don't last - unless you keep it up in that category.