It's unusual for chiller to kick in every 10 mins, showing signs that prob your chiller internal temp sensor is faulty cos tank temp shdn't rise so fast, unless you're using strong MH. Possible that your chiller was cut off when it didn't chill your tank water to 28 degrees, but the internal temp sensor read 28 degrees. That's why you think your chiller can chill the water so fast, but after 10 mins, it kicks in again. Try to get an external thermometer to measure your tank temp when your chiller cuts off at 28 degrees and see if both tally.
It's recommended to use external sensor/temp controller for accurate reading, your chiller will not kick in so often and that reduces your electricity bills as well. I think there are a few threads discussing on external temp sensor (the brand is Fox), it's very easy to use, you either DIY or get the chiller/air-con shop to make one for you at around three blue notes.
Hope this info helps.