vegas 8 pro error on long renders

G'day all

Just started using vegas 8 pro for my .mts files.
I find when I add about 20, 30 second clips and use the main concept h.264 all comes out fine after adding dissolves and slight sharpening. However if I add about 80 or 90 clips, using the same parameters, I get the message

"The following pins failed to find a connectable filter" C:\downloads\Untitled.mp4::Output

Media Type 0:
--------------------------
Unknown

AM_MEDIA_TYPE:
majortype: MEDIATYPE_Stream {E436EB83-524F-11CE-9F53-0020AF0BA770}
subtype: Unknown GUID Name {08E22ADA-B715-45ED-9D20-7B87750301D4}
formattype: TIME_FORMAT_NONE {00000000-0000-0000-0000-000000000000}
bFixedSizeSamples: 1
bTemporalCompression: 0
lSampleSize: 1
cbFormat: 0"


Any advice appreciated.

Bill