I have a project that I was working on that I created in the previous version of kdenlive. I upgraded the other day and am now seeing very jerky playback in the video monitor.

My machine is a 6 core AMD1090T with 8G of ram running Ubuntu 10.04

Am I right in saying that it is ffmpeg that controls the playback on the monitor? I have listed my version below.

FFmpeg version SVN-r0.5.1-4:0.5.1-1ubuntu1.1, libavutil 49.15. 0 / 49.15. 0
libavcodec 52.20. 1 / 52.20. 1
libavformat 52.31. 0 / 52.31. 0
libavdevice 52. 1. 0 / 52. 1. 0
libavfilter 0. 4. 0 / 0. 4. 0
libswscale 0. 7. 1 / 0. 7. 1
libpostproc 51. 2. 0 / 51. 2. 0
built on Mar 31 2011 18:59:37, gcc: 4.4.3

My project contains footage shot on a GoPro,1920x1080 @ 29.970030fps. 32 clips are used, 8.2Gib with 12.6GiB unused.
I know this is a lot of footage, I did have some issues regarding speed when I moved to a different section of the timeline
with the previous version, but left alone the basic project playback did work.

At the moment I don't use proxy clips, would I benefit from this?

I only seem to use ~1.1Gib of memory and 100-160% of CPU. Is there a way I can utilise more of my system resource?

I also see a lot of these messages during playback

[h264 @ 0x7f4f9059a7d0]mmco: unref short failure
[h264 @ 0x7f4f9059a7d0]number of reference frames exceeds max (probably corrupt input), discarding one
[h264 @ 0x7f4f9059a7d0]AVC: Consumed only 97925 bytes instead of 97928
[h264 @ 0x7f4f9059a7d0]mmco: unref short failure
[h264 @ 0x7f4f9059a7d0]number of reference frames exceeds max (probably corrupt input), discarding one
[h264 @ 0x7f4f9059a7d0]AVC: Consumed only 82171 bytes instead of 82174
[h264 @ 0x7f4f9059a7d0]mmco: unref short failure
[h264 @ 0x7f4f9059a7d0]number of reference frames exceeds max (probably corrupt input), discarding one
[h264 @ 0x7f4f9059a7d0]mmco: unref short failure

I also see a lot of these messages during playback

[h264 @ 0x7f4f9059a7d0]mmco: unref short failure
[h264 @ 0x7f4f9059a7d0]number of reference frames exceeds max (probably corrupt input), discarding one
[h264 @ 0x7f4f9059a7d0]AVC: Consumed only 97925 bytes instead of 97928
[h264 @ 0x7f4f9059a7d0]mmco: unref short failure
[h264 @ 0x7f4f9059a7d0]number of reference frames exceeds max (probably corrupt input), discarding one
[h264 @ 0x7f4f9059a7d0]AVC: Consumed only 82171 bytes instead of 82174
[h264 @ 0x7f4f9059a7d0]mmco: unref short failure
[h264 @ 0x7f4f9059a7d0]number of reference frames exceeds max (probably corrupt input), discarding one
[h264 @ 0x7f4f9059a7d0]mmco: unref short failure

Proxy clips would definitely make the playback smoother.
You might also want to try out increasing the number of threads used (settings dialog -> environment -> MLT). You shouldn't set it higher then 5 (leaving one core for an additional output thread). But be aware this feature is experimental!