What developer are you using? More than likely, what's happening is the developer exhausting although it is unusual to show that quickly.

The processing does slow when temperature goes down but you are reporting some extreme changes.

Start diagnosing this problem by finding out what developer datasheet says how many prints you can produce per given amount of stock solution. Then eliminate the variables such as temperature variance. No need to go crazy with temp accuracy. Several degree variance won't cause drastic changes.

To me, it sounds like your developer has deteriorated in storage. Then as yo printed, it reached the exhaustion state. most developers degrade fairly quickly when exposed to oxygen in air. So that may be a factor as well.