Malfunctioning in Premium Spark

When I open a document and start editing, it lets me do few changes and then, the screen gets bugged, it doesn't scroll, the screen stops totally and then in goes blank, then it bugs again, and the screen goes black, sometimes after that, it allows me to continue editing but sometimes the screen continues freeze. When this happens, the screen stops completely so I can't go home or exit the document. If i can edit again after all the steps mention prior, it only lets me change few things like a line or a bullet point so it's really slow even being a Premium member