Error -2038: Record locked by another transaction / Internal error occured

Hi, sometimes when I'm trying to update a SO via DI API, I got this error -2038. The error message varies, either: "Internal error occured" or "Record locked by another transaction". Ffor 'record locked' error, the table/field names varies, eg.

[RDR1.ItemCode][line: 7]

[RDR12.TaxId8][line: 1]

[WTR1.ItemCode][line: 1]

[RDR1.ItemCode][line: 9]

[RDR1.ItemCode][line: 3]

[RDR12.TaxId2][line: 1]

[RDR12.CityS][line: 1]

Any idea what the problem might be? FYI I'm using PL41, and this DI calls are from a centralized 'handler' VB.NET program. I've also released the instantiated BO objects via System.Runtime.InteropServices.Marshal.ReleaseComObject() (http://support.microsoft.com/kb/317109)

Thanks.