Sounds like its having root problems. You're also probably overwatering believe it or not. How big is the pot? Too small and the roots cant grow anymore, thus niether can the top of the plant. How dense is the soil? If its too dense the roots cant spread through the soil easily and it will use energy from the leaves to help the roots, thus killing the leaves. Is your soil draining well enough to allow the soil to be aired out?

Those are some questions to ask yurself when leaves turn yellow or brown..

Oh, are you using any fertilizers? If so, cut back on them. Flush the soil with clean water..