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

Is it normal behaviour? Or its depends on system configuration? If not then when it will be fixed? Or possibly this is MLT fault?
On the same PC in cinelerra and Adobe Premiere there is no such problem.

It works fine for me using NTSC DV footage. Maybe the problem is based on certain clip types, and maybe it only affects preview and not Render output. You should be more clear about that.