Arithmetic Overflow SQL

I have a stored procedure which used to work perfectly but has started to error with
"Msg 8115, Level 16, State 8, Procedure stp_UpdateTempStops, Line 9
Arithmetic overflow error converting int to data type numeric"

INSERT INTO temptbl_Stops (StartDate, StopDate, StopTime, DownTimeCode)
SELECT     TOP (100) PERCENT StartDate, StopDate, Avg(cast(DATEDIFF(s, StartDate, StopDate)/60 AS DECIMAL(4,2))) AS StopTime, DowntimeCode
FROM         dbo.tbl_MachineData
GROUP BY StartDate, StopDate, DowntimeCode
HAVING       Avg(cast(DATEDIFF(s, StartDate, StopDate)/60 AS decimal(4,2))) IS NOT NULL and DowntimeCode IS NULL and StopDate IS NOT NULL
ORDER BY StartDate

All help most welocme.....I am using SQL2012 Express