Heather, you need to get some legal stuff in place, here. You've referred to him as a "common-law husband" and common-law marriage is recognized in Montana, but there are specific factors that must be met before the courts will find a common-law marriage. Who owns/leases the house you are living in? If he has a property interest in the house he may have an interest in driving by it. You also have two kids together, which is another legitimate reason for him to be in the neighborhood.
You have your suspicions that he is "spying" on you, and you may well be correct, but without court orders in place or more alarming conduct on his part, you would probably have a hard time proving stalking.
My suggestion is that you contact a lawyer and find out what rights you have. If you feel scared by his behavior (as opposed to simply puzzled by it) then contact your local women's shelter or the National DV Hotline and get some guidance about obtaining a protective order. Based on what you've posted it's a bit "iffy" whether you would qualify for one, but best to ask an expert about it.