I match with European Jews as well but I get 8% Ashkenazi on Jtest. DNA land seems to transform my 8% Ashkenazi into 25% Mediterranian Islander for some reason. I don't know which of the two is more reliable, I guess the experts here can illustrate us.
I guess a good Y test can give you an idea about the numbers that belong to your paternal grandfather and then you can figure out your grandmother's.