My source files are always 720p, uncompressed AVIs (BGR24). My computer setup is thus:

MoBo: Asus P51D2-deluxe
CPU: Dual core processor 3.4GHZ, 64-Bit/32-Bit,
800 MHz FSB, 2MB l2 Cache, LGA 775
Memory: 1GB DDR2 533Mhz 4GB
Raid: SATA 8 port
SATA Raid controller
HDs: 7 ST3500630AS 500GB video
SATA 3 raid 5 2 mirrored
Videocard: Nvidia PCIE 16 quad 256 MB RAM
Monitor: 3 Acer 24” LCD 1920x1200
EXT HD: 500 GB

Most of the time Vegas can playback my timelines at realtime speed, which is 24fps, but occasionally (and I still don't understand what precipitates this; the dropout seems to be happening at random) the speed will drop significantly to the teens. I've been following the threads on preview speed and have tried all the settings others have suggested and nothing ever seems to work for me.

Until I lowered my Project Properties Preview size from 1280 x 720 to 720 x 405 and this seemed to be the answer. But today my playback speed once again dropped to the mid teens and I started fooling around with the preview window size, first setting it by mistake to 1980 x 720 (I wanted 1280 x 720) and my speed went back to the correct 24fps!!! The only thing wrong was that the image wouldn't fit the Preview window size (it remained smaller) -- but the aspect ratio was the correct 16:9. When I discovered my mistake of entering a completely nonsensical size of 1980 x 720 (although this never distorted the image) I started playing around with totally absurd sizes, none of which did anything wrong with the aspect ratio but some played back in the single digits, others played back correctly...

Can anyone explain this phenomenon and is there a clue somewhere in there as to what might be influencing the playback speed? I can't seem to find a pattern in any of this, yet it's apparent to me that when my playback speed drops, all I have to do to correct this is to play around with the size settings of the Project Properties.

My other Preview settings are Best (full), Scale Video to Fit Preview Window checked on, and Simulate device Aspect ratio always un-checked because it never changes anything.

I'm having the same issue.
What i think is a new update from microsoft is causing this problem.
Seems strange that this is happining to a lot of people around the same time as the update.
A reformat of your hard drive should fix this problem.

I cant think of anything else that would be causing this problem.

Any other ideas post them please.

Cheers
Simon

Thanks, Simon. I hate the thought of a HD reformatting in the middle of a huge project. The inconvenience of playing with my Preview window settings for 5 minutes every once in awhile (such as once every fortnight) is peanuts compared to the horrors of a messed up C:/ drive.

no ideas just same problem at 1080i intermediate only plays at 29.97 for a clip or two then drops to 5-6fps and thats in "draft quarter" mode. tried $400 8800 vid card and improved a little. Ever since moving to HD in vegas my system has become very unstable. I am getting 22 min timlines rendered, but many crashes and stalls during the proceedures. Will be asking some Cineform questions on their forum...