As Carol mentioned, there is a law that governs pre-existing clauses used by health insurers. It is the Health Insurance Portability Act (people refer to it as Hippa). It requires insurers to waive the pre-existing clause when a person has previously been covered with insurance through a different company. But as Carol mentioned, it does require continuous, or almost continuous, coverage. Thus is makes insurance "portable" from employer  to employer. Unfortunately I don't know if being self-employed changes any of this.

There is a person who frequents this board (Sorry I am blanking on her name right now) who works in human resources. She is very knowledgeable and I am sure she can help you if she sees your post.

As others said, be very careful what you sign up for. You probably want to check around as much as possible.