You should ask them directly instead of a bunch of lawyers on a website who have no idea and would have to pick up the phone and ask them for you so skip the middle man and get the answer.
Providing general answers are meant to help the poster to understand some complex legal concepts and in no way creates an attorney-client relationship.
What pay rate were you hired to receive? If you were only promised minimum wage then I'm not sure there is a legal issue with them voluntarily paying you more sometimes, although it could create complicated problems down the road. You should ask the owners why they do that. It may be that they are doing it to be nice since they are cutting your hours.
I completely agree with my colleagues; communicate directly with your employer. If it's a small company, they should know that your pay is fluctuating, however, it's been my experience that sometimes companies don't know exactly what they're paying their employees. Communication is key to establish and maintain a good employment relationship. Good luck.
Answer given for general advice and is not a legal opinion, which would require an analysis of the facts and circumstances as well as the applicable law and regulations.
If your employer cannot explain to you its pay policies in a plain and honest way, then that is very strange. However, the law only guarantees minimum wage*, and increases from time to time are legal (and should be welcome). If you work over 40 hours in any week, they MUST pay you 1.5 times your base rate that week unless you qualify for an exemption. (It is unlikely that you are exempt, since they are in fact paying you on an hourly basis.)
* If your employer qualifies as an employer under the Fair Labor Standards Act based upon either gross revenue annually OR they type of work that you yourself do.