what is the total volume of ur tank ?
And what is ur heat load equipment ? lighting?
the recommended flow rate for CL-280 is about 900l/h , running a 1000l/h pump with pipe friction and head loss , the flow will be seriously lower than the recommended flow rate and that will cause the chiller to kick in and out very frequently.
use a sicce extrema will be great for your chiller, was using that last time.
even a sicce nova at 1300l/hr cause the chiller to kick in/out very frequently.
although the CL 280 is rated for 280litres.. u might wanna give a 30-40 % discount to the rating as a buffer as i believe it might be overly rated .
so in all, if ur lighting is very strong ( with strong heat load 250 w MH ) , low flow through the chiller , and too big a water volume for the chiller to handle, the chiller will run non-stop.
other than that the thermostat is inaccurate , thus causing the temp to fluatuate faster than it should , getting a external thermostat will solve the prob as what bro KenYo suggested.