Mandy, Bethany went through a phase exactly the same as this last year and I was really worried. I have a friend who is a speech therapist, so spoke to her about it and she said it is perfectly normal and about 80% of children do it at some point. It is basically they are trying to get too much information out in one go and it is too much to process and they stutter.
My friend told me not to make reference to it but rather make sure that we spoke a little bit slower to Bethany and she would soon stop, which she did after about a week or two.
There is the odd occasion now when she does it when she gets excited.
Hope this helps.
xx