[Bobblehead]

Your email address is being sold.

Spammers buy huge groups of addresses. They know a percentage of them are junk, but they charge more for groups that have higher percentages of good email addresses.

So over time your spam levels go down because the batch your address is in has fewer and fewer viable addresses, so isn't used/sold as often. But sometimes the group is sold to a new spammer(sucker) or the viable emails are regrouped together and resold, and you get another spike.

Good ways to limit spam? If you get a solution you will be a billionaire.

A good email client will allow you to "train" it to help identify spam. You should still check your spam regularly to make sure there weren't any false positives, but 90% of the spam should be stopped.