Store procedure behave abnormally after each 2 or 3 days .

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ASP.NET

SQL-Server-2012

Hi Friends ,
I have a store procedure in my DB that create a unique code for me.It was working fine from last 1 year , but from last 2 week , showing unusual error after each 2 or 3 days .Strange thing is that it again start working fine after dropping and recreating procedure with same code (Error:-Failed to convert string to int).Please help me solve it .Procedure is given below

CREATE procedure [dbo].[select_purdrscode]
(@branchid int) as begin
SET NOCOUNT ON
if exists(select 1  from purdrs where branchid=@branchid)
begin
select top 1
b.branchcode +replace(CONVERT(VARCHAR(30),dateadd (minute, 330,getutcdate()),105),'-','')
+ cast((cast((REPLACE( d.drscode, left(d.drscode,11), '' ))as int)+1)as varchar(200))
 as drscode
 from PURdrs d inner join branch b
on b.branchid=d.branchid where d.branchid=@branchid
order by d.drsid desc
end
else
begin
select top 1
branchcode+replace(CONVERT(VARCHAR(30),dateadd (minute, 330,getutcdate()),105),'-','')
+'1' as drscode
 from branch   where branchid=@branchid
end
end

Thank You