I'm sorry I just don't understand what you mean.
1. If you don't have any penalties, what do you expect Google to "notice" exactly? As I said, Google will notice your no-follow tags when the page is crawled.
2. Your page rank dropping to 4's and 5's, do you mean your toolbar page rank or ranking on Google's search results? If it's a toolbar page rank - it doesn't matter one bit. If it's a ranking issue, it could be due to many things. Don't forget that you are always in competition with some else and maybe they are doing a better job, thus pushing you down the results. Penalties are much more severe than dropping a spot or two.
Anyhow, since your website has been going strong for 16 years and it appears you may not know much about SEO, I strongly urge you to do what you have been doing and don't worry about things that don't affect you. Limited knowledge is a dangerous thing. Just think that you probably run one of the longest ranking sites among the members here :)
If you can't handle the risk, go ahead and no-follow the links. Keep in mind that no one will pay you for no-follow links, unless it's a traffic deal. You may want to look into Google's AdSense to supplement your income, too. It's a form of Google-authorized paid no-follow links, but, again, it's traffic dependent.