Laurence and several others have posted about this problem.

I have experienced this several times. Here it's not on downrezzed DV but on native m2t files captured either straight from Z1 Camera Tape or from my DR60 HD recorder attached to camera.

I have always been able to workaround by creating a looped region from just before to just after the black frames, and rendering to new track, and the "missing" frames reappear.

BUT - I have just made a startling discovery - the TWO BLACK FRAMES syndrome seems to be somehow hardware dependent, and the problem only occurs in the Vegas timeline, not in Windows Media Player.

I have a clip which on my main editing PC has two black frames consistently at a specific time code.
I copied it on to a memory stick and transferred it to two other PCs.

Here are my test results. YES means Black Frames appear. NO means frames play normally:

1] Main PC (Core 2 Duo E6700):

Vegas Timeline played from Hard Drive – YES
Vegas Timeline played from Thumb Stick – YES
Windows Media Player played from Hard Drive - NO
Windows Media Player played from Thumb Stick - NO

2] Second Desktop (AMD Dual Processor 2000)

Vegas Timeline played from Hard Drive – NO
Vegas Timeline played from Thumb Stick – NO
Windows Media Player played from Hard Drive – Would not play m2t (?)
Windows Media Player played from Thumb Stick – Would not play m2t (?)

3] Vaio Laptop (Core 2 Duo T7400)

Vegas Timeline played from Hard Drive – NO
Vegas Timeline played from Thumb Stick – YES
Windows Media Player played from Hard Drive - NO
Windows Media Player played from Thumb Stick - NO

So, summarising:

Only One out of three PCs showed the black frames when playing from Hard Drive in Vegas Timeline
Apart from the laptop, symptoms were the same from Hard Drive and Thumb Stick
The black frames did not appear in WMPlayer from either thumb stick or hard drive.

Hopefully someone can make sense of this.