Could use some hardware specs.. use performance monitor to first of all find out if itīs Proc, IO or memory thatīs having a hard time.
The server has just began to slow down without warning? not more users added or some modification done to the server?
You canīt use perf monitor to find out exactly wich sp is causing you trouble but it can tell you if your hard drives are running like mad in order to scan all records in order to execute statements wich can tell you that memory could be an issue because of all the swap Or that a table or two needs more/better indexes Or maybe some sql rewriting.. puh..
I think the only way to find out wich sp thatīs causing the trouble is to run each sp in QA and then examine the execution plan generated.
Itīs a long running transaction then? from what type of system?