I went through a similar problem a few months after setting mine up and working out the settings. You've had yours for a year so it may not be quite the same.
I have my 110 gallon set up (in the UK) with 2 x 300w glass Eheim heaters. Initially I had to keep extending the heating time as it would always set off the continuous heating alarm until I was up to about 6 hours, IIRC. This was mid winter with the house heating on and this seemed OK for a while. Roll on a few months and with the house heating off the alarm started going off again. Evidently the 2 heaters were only just coping and once the ambient had dropped a little they were suddenly on continuously again.
My tank is in the kitchen which opens out on to our garden so there is a lot of air moving around, and sometimes it is quite cool.
It's possible that your heater is just about adequate for most of the time but something has changed just enough (heater, probes, ambient temp) that it is now struggling?
I intend to swap to a pair of titanium heaters soon as I was having to run 3 x 300 watt heaters for a while!
