Strange SQL Server 2005 SP Issue

We have a stored proc that accepts a date and uses that date in a filter. This procedure worked fine for a couple of weeks then started hanging. Basically, this date parameter is used in a where clause.

Our workaround: when we declare a new variable, set it equal to the parameter, then use the declared variable - that fixes it. Take away the declared variable and use the parameter instead and the proc chokes. But the paremeter and the declare variable are both datetime typed. Weird.

We are worried (partly because it worked without hanging for a while) that we are experiencing a larger issue. We are reporting off a database we denormalize and populate nightly with transactional data. A poor man's warehouse if you will, staging the data in a fast reporting format with the prep done each night. This database is wiped and re-populated each night. We have been testing this process with the same source data, so we are not growing our data at all (net result at end of each night is same data as previous day).

I think the source code here is overkill, but I enclosed the proc anyway. The way it is written below, it hangs. Replace these two uses of @AsOf in the proc with @test and the procedure almost instantly returns data. There is a third use of @AsOf that doesn't affect the issue one way or the other. This line is where the replacement fixes the problem:

--Breaks
Where (OpenedDt < DateAdd(day, 1, @AsOf) or ClosedDt < DateAdd(day, 1, @AsOf)) and (PlanType = @PlanType or @PlanType is null)

--Fixed
Where (OpenedDt < DateAdd(day, 1, @test) or ClosedDt < DateAdd(day, 1, @test)) and (PlanType = @PlanType or @PlanType is null)

If anyone has experienced this or knows some other symptoms to check for, we are all ears. This is scaring us because otherwise we have a working system on our hands and are near a delivery point. Help would be greatly appreciated.

Here is the complete proc...

ALTER PROCEDURE [dbo].[ClaimCountsByLineOfBusiness]
-- Add the parameters for the stored procedure here
@AsOf datetime,
@PlanType varchar(50)

AS
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;

DECLARE @DataFreshness datetime
Select @DataFreshness = DataFreshness From metaReportList Where StoredProcedure = 'ClaimCountsByLineOfBusiness'

If @AsOf is null
SET @AsOf = Convert(datetime, Convert(varchar(10), GetDate(), 101))

DECLARE @test datetime
SET @test = @AsOf

-- Insert statements for procedure here
Select PolicyGroup, Lines,
SUM(CASE WHEN HasPayment = 1 and OpenClosed <= 0 THEN 1 ELSE 0 END) as ClosedClaimswithPayment,
SUM(CASE WHEN HasPayment = 0 and OpenClosed <= 0 THEN 1 ELSE 0 END) as ClosedClaimswithoutPayment,
SUM(CASE WHEN OpenClosed > 0 THEN 1 ELSE 0 END) as ClaimsOpen,
Count(ClaimSummary.ClaimNumber) as ClaimsReported,
AccidentYear, ReportingYear,
--Return the value of the parameters in a friendly way (if null)
Convert(varchar(10), @Asof, 101) as AsOfParameter,
IsNull(@PlanType, 'any value') as PlanTypeParameter,
@DataFreshness as DataFreshness

FROM
(Select ClaimNumber, Max(OpenedDt) as OpenedDt, HasPayment, PolicyGroup, Lines, AccidentYear, ReportingYear
From dbo.factClaimSummary
Where (OpenedDt < DateAdd(day, 1, @AsOf) or ClosedDt < DateAdd(day, 1, @AsOf)) and (PlanType = @PlanType or @PlanType is null)
Group by ClaimNumber, HasPayment, PolicyGroup, Lines, AccidentYear, ReportingYear) as ClaimSummary

INNER JOIN

(Select ClaimNumber, SUM(CASE WHEN OpenedDt is not null THEN 1 ELSE -1 END) as OpenClosed
From dbo.factClaimSummary
Group by claimnumber) Status ON ClaimSummary.ClaimNumber = Status.ClaimNumber

Group by PolicyGroup, Lines, AccidentYear, ReportingYear
Order by PolicyGroup, Lines
END