User log in problems

[TameraD]

I have a user on my forum that logs in but after about 5 minutes gets switched to being a guest. When I view who's online I can see her IP as both her username and listed below as a guest. She is on dial up and the only way she can log back in is to delete her cookies. Is there anything that you all can think of that could be causing this? Any possible solutions?
TIA
Tam