Looks to me you've run out of resources or something, you system isn't coping with the demand you're putting on it with the project you're working on.,
You need to split the project up, make multiple pds files, x4.
x1 is a copy and back up
Open the pds using PDR
x1, split the first project, save the first third and remove all the rest. Save project as a new pds.
x1, Split the project remove the first third and the last third and save the middle as a new pds.
x1, Last third - repeat the actions as before and then save to a new project pds file.
Carry on editing in each if you wish as three separate (but re-joinable) projects or create a video off each. Because you have split at specific points in the project, you can merge the videos back together easily to complete the finish in the tracks.
Just a guess and a suggestion on what you might want to try.
Dafydd