Unusual side effects of Isophane Insulin

I am 57 years old and have been diabetic for 54 years.  I currently use bovine isophane and neutral insulins.  For some years now, I have had problems, despite my best efforts, maintaining 'normal' blood sugar levels - they vary between very high (16mmol/l +), particularly before my evening meal, and very low (sometimes below 2mmol/l).  I keep detailed records of my blood sugars and insulin doses and in recent weeks, I have noticed - at last - a pattern : every ten days or so, my blood sugars are constantly low for some 24 hours, even though I eat as much carbohydrate-rich food as I can during that period.  It is as if some of the isophane insulin has been stored somewhere in my body during those ten days and is then dumped into my system.  Is this feasible? 

Your comments and any information will be welcome.