I know it's hard, but try not to obsess over your FICO score. A 30 point fluctuation isn't unusual, and the scores will vary among the reporting agencies. I've seen them vary by more than 100 points between the agencies.
The people who are telling you to always carry at least a small balance are probably well-intentioned, but misguided and don't understand the way the scoring model works. If you're going to use credit, never carry a balance. Pay the balance in full no later than your due date.
Since part of FICO scoring depends on your use of available credit, it's not dependent on whether you PIF (which you should). Take a credit card for example. If you have a limit of $10,000, and you have $1,000 on the card, and pay it off, the company doesn't immediately report that. They all work on a schedule, and the schedule isn't dependent on what you do, it's just their reporting procedure. It is NOT based on what your balance was as of your last billing cycle, but the balance as of whenever they do their reporting, which is typically monthly, but sometimes more often. So, if you pay off your $1,000 balance, but run up $7,000, putting you at 70% of available credit, and that happens to fall during their reporting cycle, your score will drop. It doesn't mean you aren't paying your bills on time. It simply means you're using a large amount of your available credit.
You could check your FICO scores every week with all three agencies, and you'd see variations. Minor fluctuations like this are nothing to be concerned about. Major fluctuations warrant some investigation to see what caused them.