Thanks for your query.
Seasonal changes in mood and behavior are a well documented entity.Seasonal Affective Disorder is a kind of mental disorder characterized by recurrent episodes of mood changes occurring yearly during specific seasons.
The description by you points towards SAD. The changes are usually those of depression
but in your mother's case it is more likely of what we call Recurrent Unipolar Mania
as evident from the symptoms of racing thoughts and unpredictable behaviors in the background of hyper religiosity.
SAD is thought of as being due to changes in the levels of certain hormones and neurotransmitters
It is imperative that your mother be seen by a Psychiatrist
who will evaluate the seasonality
(whether these episodes occur during summer or winter),presence of depressive/other symptoms during other seasons of the year and the nature and intensity of symptoms.
SAD can be treated effectively by appropriate medications and it is also essential for her to maintain her biological rhythms such as her sleep as disruption of these are likely to trigger/worsen episodes.
Do remember that mental disorders like any other disorders are amenable to treatment
Wishing her the best in life and health.