When you say you are slipping into another rhythm, are you saying you are playing out of time or you start playing a competely different rhythm?
At your point in your learning, I think it's common to have issues with rhythm and timing. Experienced players have these problems from time to time too :).
I would suggest simplifying the rhythm. Try playing along with all down strums right on the beat. You could even try playing along with down strums only on the first beat of each measure. You can even put your guitar aside and try following the rhythm tapping the table or patting your hands on your lap. Before you can get the rhythm and timing down on your guitar, you need to hear it and feel it in general.
Good for you that you are trying to fix this now. Developing good timing and rhythm early on will really pay off down the road!