OK this is really strange.

If I run this:

SELECT dbo.MyClrFunction(sDateField)

FROM #myTempTable

it works fine on 18000 rows.

If I run this:

UPDATE #myTempTable

SET sDateField2 = dbo.MyClrFunction(sDateField)

FROM #myTempTable

I get the errors and have to drop and recreate the function and assembly to make it work again.

I really dont understand this...