The delivery method used for actions in a provisioning task can only be set when creating a template (or selecting the template in WinPE). This is at least true for v.9 (I believe all SPs). I recall reading a discussions regarding this a while back and was able to find at at the link below. Take a look at Mach6's comment. When first starting provisioning, I created two PSP delivery methods- one set to use high bandwidth usage and the other low but didn't notice any difference. I just kept the one created to use high bandwidth usage and have not heard any complaints from our networking department.
With the SWD error that you are receiving, have you looked in the provisioning logs? Also in the scheduled task itself, are there any errors/messages listed under the Status or Message columns for the computers failing?