Is the program setting up your project to match your video specs. If it is doing so correctly, then you should NOT see a yellow-orange render line above the clips in Expert view as you add them to the timeline.
Is this the case?
Also, what processor to you have and with how much RAM? What operating system are you using, and do you have the latest version of Quicktime from Apple?
What computer operating system? Is 10 and 14 on the same computer?
You clearly state in your introductory information that you have 1080p50 footage and you are manually setting the project preset to PAL AVCHD 1080p50. So that takes away any issues related to automatic setting of the project preset by the project.
(Are you putting a check mark next to "Force Selected Setting On This Project" in the new project dialog as part of your manual setting of the project preset?" "And, do your Edit Menu/Project Settings/General information agree with your manual set?)
When you import the file with the correct project preset, when it arrives at the Timeline, there should be no orange line over that content. Is that the case for you?
Assuming that it is, that is, no orange line. When that is the case, if you playback the Timeline video in the Edit area monitor (no edits yet), is the playback smooth?
Despite the fact that I manually select the setting of PAL AVCHD 1080p50 the footage is intermittently very jerky both in Quick and Expert forms
Once you edit a Timeline rendered file, it needs to be rendered again in order to get the best possible preview. Consequently, I would ask "After you edit, are you Timeline rendering the file again?"
Assuming all is Timeline rendered and the preview is not perfect, what does the export of that Timeline look like when it is played back in one of your players.
ATR, it is not necessary to invalidate my question, re project settings. As a first step, it is important to check the basics.