NSISdl default timeout for downloads

I am using the NSISdl plugin to download files off the net.
I use the /NOIEPROXY switch along with other file details

I can successfully download the openVPN installer from the parent website from my XP m/c
I try the same on Vista - its starts downloading and around 3/4 way through, it exits & gives a message: 'incomplete download'.

After trying it for the 3rd time, it went through fully.
I don't understand why the download was interrupted in the earlier attempts.

I am *not* using the /TIMEOUT switch - is there some default TIMEOUT value?
Is it a safer option to explicitly specify a TIMEOUT value?