Glitch in mp4 export but non on DVD export

Hello everyone
I am quit new with Shotcut and created my first project.
Now I want to export my movie and struggling with the right export method.

My movie starts with a transition to a text (in a second video line) and the text ends right after another transition in my first video line.

When I export the project into to DVD_pal format, the transition is running fine but the overall quality is poor.
When I export to mp4 high, the quality is good but these transition (and some others in the movie) are very glitchy.
Even if I set the quality to only 60% the transition is glitchy.

What can I improve on the mp4 export?
Is there a way to do it slower but with more focus on quality?

Thank you very much in advance!
Looking forward to every hint!

Best regards,

Maybe you can use AVI export. I had problems with mp4 too, but with AVI I got nice quality and text and transition in the right place.