Stored procedure problem

Hi,

For some reason, every 2/3 days a stored procedure on my database will start to timeout during operation. The load is normal and the indexes have been recently ( a few days ago) rebuilt. To solve the timeout i drop and then recreate the stored procedure in question. This always fixes the problem and then 2/3 days later it reoccurs. I really don't understand why thei happens, do SPs become fragmented like indexes over time and suffer perfomance degradation.

Any help would be much appreciated.