The stress is mounting as you haven't budgeted for two mortgage payments in addition to the unexpected car repairs that popped up a couple of days ago. In the beginning, when you purchased the investment property, you thought it was going to be an excellent second stream of income to help you get out of debt, but now, it's becoming a nightmare.
From this point, you've got several options, some of them completely legal, some on the side of questionable. Now, this is not legal advise and should not be interpreted as such. Read this with a light heart and seek legal help if you require further assistance.
The first solution that I'd recommend you try is to talk to the tenants. Let them know that you can't afford randomly timed rent payments and if they're unable to pay every month, on time, they'll need to move within 30 days.
In the mean time I'd recommend that you start the eviction process if you don't see any boxes 20 days into your verbal / written agreement. This process can be done on your own (which I highly recommend, especially if you want to continue your career in Real Estate landlording) or with the help of an attorney.
Some landlords have taken to the extremes of physically cutting power and water pipes to the property in an effort to smoke out the tenants. I've even heard of landlords infesting the property with pests such as bees, wasps and rats in an effort to scare them away. The most extreme case has been a property owner hiring a rouge officer to visit the property and demand immediate vacation.
I wouldn't recommend any of the previous stated methods. It's always best to stay within the confines of the law. If you or anyone you know is experiencing problems and is sick and tired of dealing with their investment property, I'll be more than happy to assist in the eviction of the problem tenants for NO CHARGE and buy the house all cash!