They can sometimes stop at the 500-600 mark although around the 1000 gm mark is far more common.

Clearly your cool and ambient temps are low. Many suggest ambient temps should never get below 75ºF. The lower core temp can cause depression in digestion there is a chance it is related to the ambient temps. I would suggest a local ambient air temp solution. I don't know what kind of set up for have but have you tried insulation, altering the ventilation paths, fluorescent lighting, or CHEs? It would be preferable to lift the ambient 5-8º if you can.