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,
Matthias

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.