Cardinality Estimation - DateTime Parameters Versus Cast?

I am trouble shooting poor performance for the query I have posted below. Here is my problem: If I run the exact query below in sql server management studio or using .NET sqlclient the query takes an average of 14 seconds to run.

However, if in .NET I take out this part of the query:
DECLARE @time_diff INT
DECLARE @start_date DATETIME
DECLARE @end_date DATETIME
SET @time_diff = 2
SET @start_date = '05/04/11 00:00:00 AM'
SET @end_date = '5/4/2011 11:59:59 PM'

And use parameters in code like this:
sqlParameter = sqlCmd.Parameters.Add(New SqlParameter("@start_date", System.Data.SqlDbType.DateTime))
sqlParameter.Value = parameters.StartDate
sqlParameter = sqlCmd.Parameters.Add(New SqlParameter("@end_date", System.Data.SqlDbType.DateTime))
sqlParameter.Value = parameters.EndDate

Then the query takes about 2 - 3 minutes to run. Also, I have noticed that it takes about 2 - 3 minutes to run, if i replace the parameter values with string date constants and run in .NET or SSMS

--AND v.call_start_time BETWEEN @start_date AND @end_date
AND v.call_start_time BETWEEN '05/04/11' AND '5/4/2011 11:59:59 PM'

I found this article [URL] on cardinality and the query optimizer and I think it has to do with my issue but it doesn't seem to make sense in my case. According to the article, casts should be executed when determining cardinality. In my case this doesn't appear to be happening. The article also says to use parameters in place of local variables - which isn't working well with .NET parameters either. Regarding using a stored procedure instead: I have a variable number of parameters, I don't know how well it would work out. Even if this is the only option, then i would still like to know exactly what the problem is.

The query:
DBCC DROPCLEANBUFFERS
DBCC FREEPROCCACHE
SET STATISTICS IO ON
DECLARE @time_diff INT
DECLARE @start_date DATETIME
DECLARE @end_date DATETIME
SET @time_diff = 2
[Code] .....