I've been making for years. However it comes out loose and my cake slides. Thoughts?

My mother made this frosting for decades because my father and brother loved it. I, personally, got tired of it by the time I was in high school. But I'm glad I found the recipe again.

TWO THINGS that should be noted. FIRST - the cooked down flour and milk can be quickly cooled by setting the sauce pan into a sink partially filled with ice and cold water. It is much faster than just putting it into a refrigerator. SECOND - the mixing of the cooled flour and milk with the remaining ingredients should be for a MINIMUM pf SEVEN (7) MINUTES. And if the recipe is doubled [my mother did this most times she made the icing and would use 1 batch to immediately frost a cake and would freeze the 2nd batch for use on a later cake] the time for mixing is also doubled.