Steps to reproduce:
1. Add a clip to a project
2. Add first part of the clip to the track 1
3. Add second part of the clip to the track 2
4. Add dissolve transition(or any other)
5. Start playing and you will see that it goes very slowly with glitches.
When I use transition between two different video files everything goes fine.
PC Configuration: CPU Q6600(2.4 x 4), 8GB RAM, HDD 400GB 7200 ext3, linux gentoo(64bit), kdenlive 0.7.6