letterbox shakes when i slow a clip down?

i've seen this in every project I've done. I shoot video with frame rate 60 fps 1080p . working on project of 25fps when i put customised moving letterbox to the clip slowed down to like 60% and export.....i look at the letterbox's on my screen and I see the letterbox jump slightly up and down. what is that?? how to avoid that jumping or shaking picture....plz help

watch video timing :
0:11 to 0:13
1:16 to 1:23
2:45 to 2:50
4:12 to 4:16

Hi Prashant,

When you use a generator to add the letterbox and then you slow down the clip and the generator, this will cause jitters on the horizontal lines.

Slow down the clips first, then add a letterbox on top of the final edit and export. That should solve the problem.

- Ronny

Helo ronny, Thanks for u r reply. i already had done what u said . so that was not d issue. i have self resolved this issue.
sorry i forget to told some minor things.
i said that i put customised moving letterbox (Means keyframes for different offsets for letterbox)
My project properties are set to 60fps
When i exported project everything is just fine.
When i encoded (from prores 422 to h.264) at 25fps at this point problem arises.
but no problem at 60fps (prores 422 to h.264)

So frame rate matters. and these shakes happening only when i added keyframes for different offsets for letterbox. this behaviour i have observed. but i don't know technical reason behind this ,why this is happening so. do u know??

Hello Rony,

Thanks for u r reply. I already done that what u said . But that was not issue. i resolved d problem.
as i said i put customised moving letterbox ( means added keyframes for different offsets in letterbox)

i forgot to said some things that, my projects properties are set to 60 fps when i exported d video everything is just fine. but when i encoded (prores422 to h.264) at 25 fps which causes that shakes but no problem for 60fps for prores422 to h.264.

I don't know technical reason y this is so happening. its happening bcd of difference in frame rates. observed this behaviour. i have 25fps project with mixed frame rates clips. so plz provide solution for how to play with such type of scenario to avoid shakes and blurry video quality.